Photography

In Plate II., fig. 14 the sensitiveness of a plate stained with homocol is shown, and it is evident that as it is sensitive throughout the visible spectrum there must be some means of cutting off by a transparent screen so much of the spectrum luminosity at different parts that every colour having the same luminosity to the eye shall be shown on a negative of equal density. When this is done the relative I'lminosities of all colours will be shown by the same relative densities. or in a print by different depths of greys. Abney devised a sensitometer which should be used to ascertain the colour of the screen that should be employed. By proper means the luminosity of the light of day coming through a red, a green, a blue and an orange glass can be very accurately measured; if k-in. squares of these coloured glasses, together with a white glass of the same area, be placed in a row and cemented on white glass, we have a colour-screen which we can make available for finding the kind of light-filter to be employed. This is readily done by reducing the luminosity of the light coming through all the glasses to that of the luminosity of the light coming through the blue glass. If the luminosity of the blue be 5 and that of the white light loo, then the luminosity of the former must be reduced to 2 o of its original value, and so with the other glasses. The luminosity of the light coming through each small glass square can be made equal by rotating in front of them a disk in which apertures are cut corresponding to the reduction required. The AgI+AgNO, on paper AgCI+AgN03 on paper AgI+--AgN03 in albumen AgI prepared in bath, treated with KI, washed, redipped in silver bath, developed with pyrogallic acid.

Grey AgBr in gelatin, developed alkaline or ferrous oxalate.. .

Orange AgBr in collodion or gelatin, alkaline ferrous oxalate or acid developer.

Green AgBr in collodion, developed ferrous oxalate. .. .. .

AgCI in collodion, excess of AgN03 or NaC1 present, ferrous citrate or acid development.

AgI+AgBr, washed from AgN03 2 10 3AgI AgBr -}- AgN03 collodion, wet plate, acid or alkaline developer FIG. I. - Spectrum Effects on Salts of Silver.

[P. = print; D. = developed; 1.e. = long exposure].

P. P.

P. D. (1.e.) D. (l.e.) D. (l.e.) D. (l.e.) D. (I.e.) P. D. (I.e.) 4? o?

E ' '[[Dcba ' 'G ' 'F H]] 5 10 15 20 25 r 30 35 Continuous Spectrum Taken With The Electric Arc.

5 15 Fluorescent Spectrum Of Eosin.

10 15 20 25 30 35 Spectrum Of Volatilized Lithium And Sodium.

10 15 20 25 30 Absorption Spectrum Of Eosin.

35 Graduation Scale On Homocol Stained " Seed " Plate.

Graduation Scale On Unstained Plate.

Impressed Continuous Spectrum.

blue glass, for instance, would not be covered by the disk at all, while opposite the white square the disk would have an aperture of an angle of 18°. When a plate is exposed behind the row of glass squares, with the light passing through the rotating disk, having the appropriate apertures for each glass, the negative obtained would under ordinary conditions, show square patches of very different opacity. A light-filter of some transparent colour, if placed in the path of the light, will alter the opacities, and eventually one can be found which will only allow such coloured light to be transmitted as will cause all the opacities in the negative to be the same. As the luminosities of the white light passing through the glasses are made equal, and as the photographic deposits are also rendered equal, this light-filter, if used in front of the camera lens, will render all coloured objects in correct monochrome luminosity. Another plan, based on the same principles, is to place segments of annuluses of vermilion, chrome yellow, emerald green, French blue and white on a disk, and to complete the annuluses with black segments, the amount of black depending on the luminosity of the pigments, which can be readily measured. When the disk is rotated, rings of colour, modified in brightness by black, are seen, and each ring will be of the same luminosity. As before, a screen (light-filter) to be used in front of the lens must be found which will cause the developed images of all the rings to appear of equal opacity. It must be remembered that the light in which the object is to be photographed must be the same as that in which the luminosity of the glasses or pigments is measured.

The salts most usually employed in photography are the bichromates of the alkalis. The result of spectrum action is confined to its own most refrangible end, commencing in the ultra-violet and reaching as far as in the solar spectrum. Fig. 2 shows the relative action of FIG. 2. - The top letters have reference to the Fraunhofer lines; the bottom letters are the initials of the colours. The relative sensitiveness is shown by the height of the curve above the base-line.

the various parts of the spectrum on potassium bichromate. If other bichromates are employed, the action will be found to be tolerably well represented by the figures. No. 1 is the effect of a long exposure, No. 2 of a shorter one. It should be noticed that the solution of potassium bichromate absorbs those rays alone which are effective in altering the bichromate. This change is only possible in the presence of organic matter of some kind, such as gelatin or albumen.

This seems to be continued into and below the red, the blue rays, however, are the most effective. The action of light on this body is to render it less soluble in its usual solvents.

The commonest ferric salt in use is the oxalate, by which the beautiful platinotype prints are produced. We give this as a representation (fig. 3) of H G F E Dcba Fig. 3. - Same description as for fig. 2.

the spectra obtained on ferric salts in general. Here, again, we have an example of the law that exists as to the correlation between absorption and chemical action. One of the most remarkable compounds of iron is that experimented upon by Sir J. Herschel and later by Lord Rayleigh, viz. ferrocyanide of potassium and ferric chloride. If these two be brushed over paper, and the paper be then exposed to a bright solar spectrum, action is exhibited into the infra-red region. This is one of the few instances in which these light-waves of low refrangibility are capable of producing any effect. The colour of this solution is a muddy green, and analysis shows that it cuts off these rays as well as generally absorbs those of higher refrangibility.

The salts of uranium are affected by light in the presence of organic matter, and they too are only acted upon by those rays which they absorb. Thus nitrate of uranium, which shows, too, absorption-bands in the green blue, is affected more where these occur than in any other portion of the spectrum.

Some salts of mercury, gold, copper, lead, manganese, molybdenum, platinum, vanadium, are affected by light, but in a less degree than those which we have discussed. In the organic world there are very few substances which do not change by the continuous action of light, and it will be found that as a rule they are affected by the blue end of the spectrum rather than by the red end (see Photochemistry).

The following table gives the names of the observers of the action of light on different substances, with the date of publication of the several observations. It is nearly identical with one given by Dr Eder in his Geschichte der Photo-Chemie. 'No.3 ' VI Gyor Effect of Hydrogen Peroxide on Sensitive Plates. - Dr W. J. Russell made a series of experiments on the effect of exposure of sensitive plates to the action of vapours and gases for long periods. It has long been known that contact of plates with such substances as wood caused a sensitive surface to show " fog " on development. By a somewhat exhaustive series of experiments, Russell showed that the probable cause of this fog is hydrogen peroxide, since substances which favoured its formation produced the same effect. This is somewhat remarkable, as this same substance will completely destroy the effect that light has had on a sensitive plate; indeed, it affords one way of destroying a light image on a sensitive collodion plate. The experiments of Russell give a warning to store exposed plates for brief periods. It appears that negatives wrapped in paraffin paper are secure from this danger.

In order to employ photography for the measurement of light it was necessary that some means should be devised by which the opacity of the deposit produced on the development of a plate could be determined. It is believed that in 1874 the first attempt was made by Sir W. Abney to do this. In the Phil. Mag. he showed how density could be measured by means of an instrument, the diaphanometer, he had devised, in which transparent 40 20 10 5 black wedges were used to make matches between the naked light and the same light after passing through the photographic opacity that had to be measured. In 1887, owing to the perfecting of the rotating sectors, which could be made to increase or diminish the apertures at pleasure during its rotation, the measurement of opacities became easy. The Rumford method of comparing the light through the deposit with the naked beam, using the sectors to equalize the illumination, was adopted, the deposit being placed between the light and the screen, the comparison light being a beam reflected from the same light on to the screen.

Owing to the fact that photographic deposit scatters light more or less, the opacities measured by this plan were slightly greater than was shown when such opacities were to be used for contact printing. The final plan adopted by Abney was to place the part of the plate carrying the deposit to be measured behind a screen constructed as above. C D (fig. 4) is a C dull black card with anaperture cut in it which may be of any desired shape.

A B This aperture was covered with trans parent paper, as was also a portion B, the same size as A, but pasted on the black card itself. Light thrown from D behind A would be matched with light FIG. 4. thrown on to B from the front when a rod in the path of this last beam was made to prevent this light falling on A. When a portion of a plate bearing a deposit was placed behind and close to A, the light thrown on B had to be diminished by the sector till the two squares appeared equally bright and the aperture of the sector was noted and compared with that required when the deposit was removed.

With this screen accurate measures of printing densities can be made, and it can also be used in the determination of the comparative photographic brightness of the light issuing from different objects. For instance, the relative brightness of the different parts of the corona as seen in a total eclipse can be readily determined if a " time scale " of gradation is impressed on the plate on which it is taken. Both scale and streamer can then be enlarged optically and thrown on the part of the screen A. The measures of the streamer densities can then be directly compared with the densities of the scale and the relative " photographic " brightness of the different parts of the streamer be ascertained by comparison with this scale also.

The same method of measurement was adopted in ascertaining quantitatively the sensitiveness of the spectrum of ordinary plates and of plates in which dyes are present. The figures on Pl. IV show reproductions of plates which were exposed to the spectrum. No. 1 is a continuous spectrum taken with the electric light; no. 7 is an impressed continuous spectrum; no. 8 shows the bright lines of metals; no. 3 the line spectrum of volatilized lithium and sodium to indicate the position of the spectrum colours. Nos. 4 and 2 are the absorption and fluorescent spectra of eosin. No. 5 is the graduation scale formed by a bromogelatin " Seed " plate stained with homocol, a cyanine derivative sensitive to the red; no. 6 is a similar scale formed by an unstained plate. The small numbers placed below the different bands show an empiric scale which is made to apply to each of them. The first step is to measure ei of the spectrum FIG. 5.

the opacity of the gradation scale, next the opacity of the continuous spectrum at the various numbers of the empiric scale, and also the opacity of the other bands at the same scale numbers. The continuous spectrum will give the sensitiveness of the plate to the different parts of the spectrum when the measures of its different opacities are compared with those of the scale of gradation, and a curve of sensitiveness can be plotted from these comparisons. It is evident that the measures of the other two bands will give us information as to the fluorescence and the absorption of the eosin. Fig. 5 shows the curve of opacity of the image of the spectrum at its different parts, and also the curve of sensitiveness of the plate to the different parts of the spectrum. This last is derived from a comparison of the measured densities with those of the gradation scale.

The first attempt that was made to ascertain the rapidity of a plate was by Abney (Phil. Mag. 1874), who demonstrated that within limits the transparency of deposit varied as the logarithm of the exposure.

The last formula has been accepted for general use, though it is believed that it is not absolutely correct, though very approximately true and sufficiently near to be of practical value. This belief is based on the further researches described below.' In 1888 Sir W. Abney pointed out that the speed of a plate could be determined by the formula T = E-,u(10g E+C) 2, where T is the transparency, E is the exposure (or time of exposure X intensity of light acting), and C a constant. If the abscissae (exposures) are plotted as logarithms, the curve takes the same form as that of the law of error, which has a singular point, a tangent through which lies closely along the curve and cuts the axis of Y at a point which has a value of 2/ 1 / E. If the total transparency be unity, this ordinate has a value of 1.212, the singular point having a value of o606. The ordinate of the zero point of the curve will be where the tangent to the singular point cuts the line drawn at 1.212. The difference between the measurements of this zero point for two kinds of plates (i.e. C in the formula) from the points in the abscissae marking the same exposure, will give the relative sensitiveness of the two plates in terms of log x 2. In 1890 Hurter and Driffield (Journ. Soc. Chem. Ind. Jan. 19, 1891) worked out a less empirical formula connecting the exposure E with the density of deposit, which in an approximate shape had the form D =ylog(E/i), where D is the density of deposit (or log 1 /T), i the " inertia " of the plate, T the transparency of the deposit. In the customary way a small portion of a plate was exposed to a constant light at a fixed distance and for a fixed time, and another small portion to the same light for double the time, and so on. By measuring the densities of the various deposits and constructing a curve, a large part of which was approximately a straight line, it was found possible, by the production of the straight portion to meet the axis of X, to give the relative sensitiveness of different plates by the distance of the intersection from the zero point L. (See also Exposure Meters, below, under § I, Apparatus.) Effect of Temperature on Sensitiveness. - In 1876 Abney showed that heat apparently increased, while cold diminished, the sensitiveness of a plate, but the experiments were rather of the qualitative than the quantitative order. In 1893, from fresh experiments,' he found that the effect of a difference in temperature of some 40° C. invariably caused a diminution in sensitiveness of the sensitive salt at the lower temperature, a plate often requiring more than double the exposure at a temperature of about - 18° C. than it did when the temperature was increased to +33° C. The general deduction from the experiments was that increase in temperature involved increase in sensitiveness so long as the constituents of the plate (gelatin, &c.) were unaltered. Sir James Dewar stated at the Royal Institution in 1896 that at a temperature of - 180° C. certain sensitive films were reduced in sensitiveness to less than a quarter of that which they possess at ordinary temperatures. It appears also, from his subsequent inquiry, that when the same films were subjected to the temperature of liquid hydrogen (- 252° C.) the loss in sensitiveness becomes asymptotic as the absolute zero is approached. Presumably, therefore, some degree of sensitiveness would still be preserved even at the absolute zero.

Effect of Small Intensities of Light on a Sensitive Salt.3 - When a plate is exposed for a certain time to a light of given intensity, it is commonly said to have received so much exposure (E). If the time be altered, and the intensity of the light also, so that the exposure (time X intensity) is the same, it was usually accepted that the energy expended in doing chemical work in the film was the same. A series of experiments conducted under differing conditions has shown that such is not the case, and that the more intense the light (within certain limits) the greater is the chemical action, as shown on the development of a plate. Fig. 6 illustrates the results obtained in three cases. The exposure E is the same in all cases. The curves are so drawn that the scale of abscissae 1 Those applicable to the correction of star magnitudes as determined by photography have been verified and confirmed by Schwarzchild, Michalke and others.

2 Abney, Proc. Roy. Soc. 1893.

Abney, Proc. Roy. Soc. 1893, and Journ. Camera Club, 1893.

50 60 70 is the intensity of the light in powers of - 2, and the ordinates show the percentages of chemical action produced. If the chemical action remained the same when the intensity of 'light was reduced, E remaining the same, each of the curves would be shown as a straight line at the height of ioo, which is the transparency of deposit with the unit of light. As it is, they show diminishing percentages as the light intensity is diminished.

3 FIG. 6.

Thus, when the intensity of the light is reduced to T of the original, and the time of exposure is prolonged 64 times, the useful energy expended on a lantern plate is only s o % of that expended when the light and time of exposure are each unity. In the cases to which the diagram refers, the light used was a standard amyl acetate lamp, and the unit of intensity taken was this light at a distance of 2 ft. from the plate, and the unit of time was 10 seconds. The lamp being moved to 16 ft. from the plate, gave an intensity of 6 1 4 the unit, and the time of exposure had to be increased to 640 seconds, so that E was the same in both cases. Further, it was found that when the times of exposure on different parts of the plate were successively doubled, light at a fixed distance being used for one series, and altered for a second series, the slopes of the curves of transparency (i.e. the gradation) were parallel to one another. This investigation is of use when camera images are in question, as the picture is formed by different intensities of light, not very different from those of the amyl acetate lamp, the time of exposure being the same for all intensities. The deductions made from the investigation are that with a slow plate the energy expended in chemical action is smaller as the intensity is diminished, while with a quick plate the variation is much less. As a practical deduction, we may say that to obtain proper contrast in a badly lighted picture it is advisable to use a slow plate.

Another investigation was made as to the effect of very intense light on sensitive surfaces. In this case a screen of step-by-step graduated opacities was made use of, and plates exposed through it to the action of lights markedly differing in intensity, one being that of the amyl acetate lamp, another that of the arc light, and a third the light emitted from the spark of a Wimshurst machine. The exposures were so made that one of the opacities produced on the plate from exposure to each source of light was approximately the same. The unit of intensity of light is, of course, in each case widely different. The slope of the curve due to the spark light is less steep than that due to the arc light, and the latter, again, is much less steep than that due to the amyl acetate lamp. A further investigation was made of the effect of increasing the time of exposure when the intense light was diminished, and it was found that with all plates the useful chemical energy acting on a plate was least with the most intense light, but increased as the intensity diminished, though the time was correspondingly increased. This is the reverse of what we have recorded as taking place when a comparatively feeble light was employed. Further, it was proved that the variation was greatest in those plates which are ordinarily considered to be the most rapid. It follows, therefore, that there is some intensity of light when the useful chemical energy is at a maximum, and that this intensity varies for each kind of plate.

The same investigator has shown that, if a total exposure is made up of intermittent exposures, the chemical action on a sensitive salt is less than it is when the same exposure is not intermittent. It was also proved that the longer the time of rest between the intermittent exposures (within limits) the less was the chemical action. We may quote one case. Exposures were first made to a naked light, and afterwards to the same light for six times longer, as a rotating disk intervened which had 12 apertures of 5° cut in it at equal intervals apart, and 720 intermittent exposures per second were given. The plate was moved to different distances from the light, so that the intensity was altered. The apparent loss of exposure by the intervention of the disk increases as the intensity diminishes, the ratios of the chemical energy usefully employed of the naked light exposure to that of the intermitting exposures being: - For intensity r I to 815 5 „ A I „ .423 I „ 370 These results appear to be explicable by the theoretical considerations regarding molecular motion.

It has been a subject of investigation as to whether the gradation on a plate is altered when exposures are made to lights of different colours; that is to say, whether the shades of tone in a negative of a white object illuminated by, say, a red light, would be the same as those in the negative if illuminated by a blue light. Abney 1 announced that the gradation was different; and, quite independently, Chapman Jones made a general deduction for isochromatic plates that, except with a certain developer, the gradation was steeper (that is, the curve shown graphically would be steeper) the greater the wave-lengths of the light to which the sensitive salt was subjected. For plates made with the ordinary haloid salts of silver Chapman Jones's deduction requires modification. When monochromatic light from the spectrum is employed, it is found that the gradation increases with wave-lengths of light which are less, and also with those which are greater, than the light whose wave-lengths has a maximum effect on the sensitive salt experimented with. Thus with bromo-iodide of silver the maximum effect produced by the spectrum is close to the blue lithium line, and the gradation of the plate illuminated with that light is less steep than when the light is spectrum violet, green, yellow or red. From the red to the yellow the gradation is much the steepest. Whether these results have any practical bearing on ordinary photographic exposures is not settled, but that they must have some decided effect on the accuracy of three-colour work for the production of pictures in approximately natural colours is undoubted, and they may have a direct influence on the determination of star magnitudes by means of photography.

Ives's Process. - A practical plan of producing images in approximately the true colours of nature has been devised by preparing three positives of the same object, one 1 Proc. Roy. Soc., ?

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/ 5 6 FIG.

.) 90 ro t ' '41 ' 'V ' '`1 g illuminated by a red, the other by a green, and the third by a blue light; the images from these three transparencies, when visually combined, will show the colours of the object. This plan was scientifically and practically worked out by F. E. Ives of Philadelphia, though in France and elsewhere it had been formulated, especially by Hauron Du Cros.

The following description may be taken as that of Ives's process: by the trichromatic theory of colour-vision every colour in nature can be accounted for by the mixture of two or three of the three-colour sensations, red, green and blue, to which the eye is supposed to respond. Thus a mixture of a red and green sensation produces the sensation of yellow; of a green and blue, that of a blue-green; of red and blue, that of purple; and of all three, that of white. For the sensations we may substitute those colours which most nearly respond to the theoretical sensations without any material loss of purity in the resulting sensation. We must take the spectrum of white light as the only perfect scale of pure colours. It has been proved that the red sensation in the eye is excited by a large part of the visible spectrum, but with varying intensities. If, then, we can on a photographic plate produce a developed image of the spectrum which exactly corresponds in opacity and position to the amount of red stimulation excited in those regions, we shall, on illuminating a transparent positive taken from such a negative with a pure red light, have a representation of the spectrum such as would be seen by an eye which was only endowed with the sensation of red. Similarly, if negatives could be taken to fulfil the like conditions for the green and for the blue sensations, we should obtain positives from them which, when illuminated by pure green and blue light respectively, would show the spectrum as seen by an eye which was only endowed with a green or a blue sensation. Evidently if by some artifice we can throw the coloured images of these three positives on a screen, superposing them one over the other in their proper relative positions, the spectrum will be reproduced, for the overlapping colours, by their variation in intensity, will form the colours intermediate between those used for the illumination of the positives. For the purpose of producing the three suitable negatives of the spectrum, three light-filters, through which the image has to pass before reaching the photographic plate, have to be found. With all present plates these are compromises. Roughly speaking, the screens used for taking the three negatives are an orange, a bluish-green and a blue. These transmit those parts of the spectrum which answer to the three sensations. When these are obtained an image of a coloured object can be reproduced in its true colours.

Abney devised sensitometers for determining the colours of the screens to be placed before the lens in order to secure the threecolour negatives which should answer these requirements. Their production depends upon the same principles indicated as necessary for the correct rendering in monochrome of a coloured object. When the sensitometer takes the form of glasses through which light is transmitted to the plate, the luminosities of the coloured lights transmitted are determined, and also their percentage composition in terms of the red, green, and blue lights, and thence are deduced the luminosities in terms of red, green and blue. For ascertaining what screen should be used to produce the red negative the luminosity transmitted through each glass is so adjusted that the luminosity of the red components in each is made equal by rotating a disk with correct apertures cut out close to the row of glasses. This gives a sensitometer of equal red values. A coloured screen has to be found which, when placed in front of the lens, will cause the opacities of the deposit on the plate, corresponding to each square of glass, to be the same throughout. This is done by trial, the colour being altered till the proper result is obtained. In a similar way the " green " and " blue " screens are determined. Coloured pigments rotating on a disk can also be employed, as indicated in the paragraph on the correct rendering of colour in monochrome.

As to the camera for the amateur, whose plates are not as a rule large, all of the three negatives should be obtained on one plate, since only in this way can they be developed and the densities increased together. (For commercial work the negatives often cannot be taken on the same plate, as it would make the plate too large to manipulate.) The camera may be of an ordinary type, with a repeating back, bringing successively three different portions of the plate opposite the lens. It is convenient to have a slide, in front of which a holder containing the three screens can be fixed, which will then be close to the plate; such a one has been devised by E. Sanger-Shepherd. The light passes through them one by one as the plate is moved into the three positions. The three exposures are given separately, after which the plate is ready for development. The three separate exposures are, however, a source of trouble at times, particularly in the case of landscapes, for the lighting may vary and the sky may have moving clouds, in which case the pictures would show variations which should not exist. Sanger-Shepherd has a " one-exposure " camera by which the three images are thrown side by side on the plate. Thus any movement in the picture affects all three negatives alike. Abney has also introduced a " one-exposure " camera which takes in a larger angle than that of Sanger-Shepherd. The next point is the exposures which should be given through each screen. This can be done by placing in front of the plate and extending its whole length a scale of gradation through which the light coming from a sun-illuminated white card passes, as well as through the screens. In the case of the three-exposure camera the times of exposure are varied till the densities of the image of the gradation appear the same in each of the three images. In the case of the one-exposure camera, the light reaching the plate through the screens is altered by cutting off with a shutter more or less of the lens used. As the plates employed for the purpose of the three-colour negatives must be sensitive to every colour, the ordinary dark-room light should be most cautiously used. If used at all, it should be very feeble and development must be carried out in a dish with a cover to it. The plate is manipulated in the usual way.

Joly's Process. - Professor J. Joly, of Dublin, in 1897 introduced a colour process by which an image in approximately natural colours could be thrown upon a screen by an optical lantern, only one transparency being employed, instead of three, as in the Ives process. A " taking " screen was ruled with alternating orange, blue-green and blue lines ob - to o ?- in. apart, touching one another and following one another in the above order. When such a screen was placed in front of a sensitive plate in the camera, and exposure made to the image of a coloured object, there were practically three negatives on the same plate, each being confined to the area occupied by lines of the same colour. The shades of colour and the depth of the colours used in ruling depended on the brand of plate. When a perfect triune negative was obtained, a transparency was made from it, and in contact with this was placed a screen ruled with lines the same distance apart, but of the colours corresponding to the three colour sensations, namely red, green and blue. The red lines were made to fall on the image taken through the orange lines, the green on that of the blue-green, and the blue or violet on that of the blue. On the screen there are practically three differently coloured images shown by one transparency. The eye blends the different colours together and a picture is seen in approximately' the correct colours of the original.

A very remarkable process, founded on J. Joly's process, was introduced in 1907 by A. Lumiere et ses Fils of Lyons. Starch grains of very minute size, some of which were dyed with a red stain, a second portion with a green, and a third portion with a blue, are mixed together in such proportions that a fine layer of them appears grey when viewed by transmitted light. Under a magnifying glass the grains are coloured, but owing to the want of focus in the eye the colours blend one with the other. Such a layer is embedded on the surface of a glass plate in a waterproof vehicle, and a film of sensitive emulsion held in situ in some material, the composition of which has not been published, covers this layer. When such a plate is placed in the camera, with the back of the plate next the lens, the light passes through the coloured granules, and again we have three negatives on one plate, but instead of each negative being represented by lines as in the Joly process they are represented by dots of silver deposit. Owing to the way in which the threecoloured film is prepared, it is evident that a positive taken from such a negative could not be backed with granules of the right colour, as the granules are placed at random in the layer. Lumiere, to overcome this difficulty, converted the negative into a positive in a very ingenious way. The plate was developed with pyrogallic and ammonia in the usual way, but instead of fixing it it was plunged into a solution of potassium permanganate and sulphuric acid. This dissolved all the silver that had been' deposited during development and left a film of unaltered silver salt. On looking through the plate the colours of the coloured layer coming through the different dots where the silver was at first deposited appeared in view, and the image was the image in colour of the object photographed. The plate after being washed was taken into the light and redeveloped with an alkaline developer, which converted the sensitive salt of silver to the metallic state. The image now consisted of black particles of silver and the coloured image. The plate was next fixed in hyposulphite of soda to remove any unreduced silver salt that might be left, and the picture after washing was complete. The coloured image so obtained is a very close representation of the true colours, but as the " taking " screen is the same as the " viewing " screen some little variation must result.

Ives was the first to show that a transparency displaying approximately all the colours in nature could be produced on the same principles that underlie the threecolour printing. This he effected by printing each of the three negatives, produced for his triple projection process as already described, on gelatine films sensitized by bichromate of potash. Each of the three transparent films was dyed with a colour complementary to the colour of the light which he transmitted through the positives when used for projection. Thus the " red " positive he dyed with a blue-green dye, the " green " positive with a purple dye, and the " blue " positive with a yellow dye. These three films, when superposed, gave the colours of the original object. Sanger-Shepherd has made the process a commercial success (see Process) and produces lantern slides of great beauty, in which all colours are correctly rendered. Instead of using a dye for the " red " transparency, he converts the silver image of a positive image into an iron salt resembling Prussian blue in colour. (W. DE W. A.) II.-Photographic Apparatus Photographic apparatus consists essentially of the camera with lens and stand, lens shutters, exposure meters, prepared plates for the production of negatives or transparencies, sensitive papers and apparatus for producing positive prints, direct or by enlargement. Besides these there are many subsidiary accessories.

Since the introduction of highly sensitive dry plates and their extended use in hand cameras, the art and practice of photography have been revolutionized. Numerous special forms of apparatus have been created suitable for the requirements of the new photography, and their manufacture and sale have become important industries. The value of the exports of photographic materials from the United Kingdom in 1906 was £22,716. The most important improvement has been in the construction of anastigmatic lenses, which, having great covering power, flatness of field, and freedom from astigmatism, can be worked with very much larger apertures than was possible with the earlier forms of rectilinear or aplanatic lenses. The increased rapidity of working thus gained has rendered it easy to photograph objects in very rapid motion with great perfection. This has encouraged the construction of the very light and compact hand cameras now so universally in use, while, again, their use has been greatly simplified by improvements in the manufacture of sensitive plates and films and the introduction of light, flexible, sensitive films which can be changed freely in daylight. The introduction in 1907 of Messrs Lumiere's " Autochrome " process of colour photography has also been a great advance, tending to popularize photographic work by the facility it offers for reproducing objects in the colours of nature.

The Camera. Historical. - The camera obscura (q.v.) was first applied to photographic use by Thomas Wedgwood between 1792 and 1802. No description of his camera is available, but it was probably one of the sketching cameras then in use. In 1812 W. H. Wollaston found that by using a meniscus lens with a concave surface towards the object and the convex towards the screen, a diaphragm being placed in front, the projected image of the camera obscura was greatly improved in sharpness over a larger field. The first photographic lenses made by V. and Ch. L. Chevalier in Paris (1830-1840) were on this principle. The photographic camera in its simplest form is a rectangular box, one end of which is fitted to carry a lens and the opposite one with a recess for holding the focusing screen and plate holders, these ends being connected by a rigid or expanding base-board and body, constructed to keep out all light from the sensitive plate except that passing through the lens. In 1816 Joseph Nicephore Niepce, of Chalon-sur-Saone, for his photographic experiments made a little camera, or artificial eye, with a box six inches square fitted with an elongated tube carrying a lenticular glass. There are now in the Chalon Museum cameras of his with an iris diaphragm for admitting more or less light to the lens; some with an accordion bellows, others with a double expanding rigid body for adjusting the focus. The iris diaphragm was adopted later by Chevalier for his photographic lenses. In 1835 W. H. Fox Talbot constructed simple box cameras for taking views of his house on sensitive paper, and claimed them as the first photographs of a building (Phil. Mag. 18 39, 1 4, p. 206). Fr. von Kobell and C. A. Steinheil, early in 1839, made a camera with an opera glass lens for taking landscapes on paper. Later in 1839 J. W. Draptr successfully used a camera for his daguerreotype experiments made of a spectacle lens, 14 in. focus, fitted into a cigar box. He also used a camera fitted with a concave mirror instead of a lens. Similar cameras were constructed by A. T. Wolcott (1840) and R. Beard (1841) for reversing the image in daguerreotype portraits. They have also been recommended by V. Zenger (1875) and D. Mach (1890) for scientific work.

L. J. M. Daguerre's camera, as made by Chevalier in 1839 for daguerreotype, was of Niepce's rigid double body type, fitted with an achromatic meniscus lens with diaphragm in front on Wollaston's principle, the back part with the plate moving away from the lens for focusing, and fixed in its place with a thumbscrew. This expanding arrangement enabled lenses of different focal lengths to be used. With modifications cameras of this type were in use for many years afterwards for portrait and studio purposes. For work in the field they were found inconvenient, and many more portable forms were brought out, among them G. Knight's and T. Ottewill's single and double folding cameras (1853), made collapsible with hinges, so as to fold on to the base-board. Cameras with light bodies made of waterproof cloth, &c., also came into use, but these were superseded by cameras with collapsible bellows-body of leather, which, invented by Niepce, were used in France, in 1839, by Baron A. P. de Seguier and others for daguerreotype. The first record of them in England is, apparently, J. Atkinson's portable stereoscopic camera of parallel-side bellows form (Ph. Journ. 1857, 3, p. 261), which was soon followed by C. T. H. Kinnear's lighter conical form, made by Bell of Edinburgh (Ph. Journ. 1858, 4, p. 166). They have since been made in various patterns, conical, oblong and square, by P. Meagher, G. Hare and others, and are still, in modified forms, in general use as studio, field or hand cameras. When wet collodion plates were used many cameras were fitted with arrangements for developing in the field.

Information on these and other early cameras will be found in the photographic journals, in C. Fabre's Traite encyclopedique de photographie, vol. i., and in J. M. Eder's Ausfiihrliches Handbuch der Photographic, 2nd ed., vol. i., pt. ii.

The distinctive feature of present day photography is the world-wide use of the hand camera. Its convenience, the ease with which it can be carried and worked, and the remarkably low prices at which good, useful cameras of the kind can be supplied, concurrently with improvements in rapid sensitive plates and lenses, have conduced to this result. It has also had a valuable educational influence in quickening artistic perception and scientific inquiry, besides its use in depicting scenes and passing events for historical record. Small portable cameras had been made by B. G. Edwards (1855), T. Scaife (Pistolgraph, 1858), A. Bertsch (1860), T. Ottewill (1861), and others, but it was not until rapid gelatin dry plates were available in 1881 that T. Bolas brought out his " detective " camera (Ph. Journ. 1881, p. 59). It consisted of a double camera (one as finder, the other for taking the picture) enclosed in another box, suitably covered, which also contained the double-plate carriers and had apertures in front of the viewing and taking lenses. In another form the finder was omitted. A month later A. Loisseau and J. B. Germeuil-Bonnaud patented an opera glass camera. Various forms of portable magazine cameras followed, among them A. Pumphrey's " Repeating Camera " (1881), W. Rouch's " Eureka" (1887), R. Krugener's camera (book form, 1888), and others in collapsible or box forms disguised as books, watches, &c., but they did not come into general use before 1888, when the Eastman Company of Rochester, U.S.A., brought out their very portable roll-film cameras, now known under the trade name of " Kodak." The manufacture of these and other light hand cameras has since become a very important and flourishing industry in Great Britain, Germany, France and the United States. It is noteworthy that the most modern form of hand camera, the reflex, goes back to an early type of portable camera obscura, figured by Johann Zahn in 1686, in which a mirror was used for reflecting the image on to a horizontal focusing screen, at the same time reversing it. The first photographic camera on this principle was T. Sutton's (1860), which has served as a basis for many subsequent developments. A. D. Loman's (1889) and R. Krugener's (1891) were early examples of the hand camera type, but great improvements have since been made.

Modern cameras differ so much in details of improved construction that only a few of the more important requirements can be noticed. A camera should be well and strongly made of seasoned wood or of metal, perfectly rigid when set up, to avoid any shifting of the axis of the lens in respect to the sensitive plate. The front and back of the camera should normally be vertical and parallel, and the axis of the lens perpendicular to the centre of the plate, but arrangements are usually made by vertical and lateral adjustments on the camera front for raising the lens to take in less foreground or vice versa, or for moving it right or left, the latter becoming a vertical movement when the camera has to be turned on its side. In the Adams " Idento " camera the lens and finder can be rotated together on the rising front according as the camera is used horizontally or vertically, the finder showing in either case the identical view projected on the plate. The best modern field cameras are fitted with a swing-back or swing-front and sometimes with both. A swing-back is necessary for bringing back the plate to the vertical position, so as to prevent convergence of vertical lines, when the camera has to be tilted. A rising swing-front, in which the lens is tilted, answers the same purpose, provided the camera is kept level. If further tilting is necessary, when taking high buildings &c., the swing-back and front may both be required, but must be kept vertical and parallel and the effect is that of an abnormal rising front. Many modern cameras are fitted with a double rising front. The vertical and side swings are also useful for equalizing the definition of objects at different distances from the camera, but they alter the perspective. These swing-movements should preferably be round the central horizontal or vertical axis of the back or front, but are frequently effected by simple inclination of the back or lens front on a hinge. When the rising front is used a lens of extended covering power is desirable, and it may be necessary to stop it down to obtain good definition over the extended area of the picture. A slight inclination of the lens may also be useful in readjusting the focus. The camera and plate carriers must be perfectly light-tight and all inner bright surfaces made dead black to prevent reflections from bright spots being thrown on the plate. The black varnish used, preferably of shellac and lampblack in spirit, must have no deleterious effect on the plates. Although the weight and bulk are increased it is convenient to have the camera square and fitted with a reversible back, so that the greatest length of the plate may be horizontal or vertical, as desired. Many cameras are fitted with revolving backs to be used in either position. In some French cameras the back part of the camera with the bellows is reversible, to be used upright or horizontal.

The earlier cameras were focused by drawing out the back and clamping it with a thumb-screw working in a slot in the base-board. When bellows cameras were introduced they were focused by an endless screw, and these are still used for large copying cameras. Most modern cameras are fitted with rack and pinion movements working either in front or at the back of the camera or both. Many hand cameras, requiring to be brought to focus at once, are fitted with studs (infinity catches) which fix the front in focus for distant objects, nearer distances being noted on an engraved scale attached to the base-board. Such scales should be verified by measurement. In hand cameras with fixed infinity focus, the necessary adjustments for distance of near objects are made on the lens mount. The focusing screen may be ruled with parallel cross lines for purposes of measurement, and as a check on the verticality of the camera when photographing buildings or other objects with vertical lines. The distance of the lens from the focusing screen and from the sensitive plate in the dark slide must coincide exactly. This can be tested by measurement or by focusing a bright, well-defined object on the screen and then on a groundglass plate placed in each of the slides to be examined. A level or other means of showing that the camera is level and the plate vertical should be attached to the camera, also a view meter or finder, showing the exact extent of the picture on the focusing glass. In the view meter the picture is viewed directly through a pin-hole mounted at the back of the camera as it appears in a frame with cross wires on the rising front, adjusted to the size of the plate and the focus of the lens. Finders are practically small reflex cameras, and a reduced image is seen reflected from a mirror or prism. A rectangular concave glass mounted on the camera is also a convenient form, it can be combined with a mirror for vertical observation, and in Watson's new form is also arranged as a level and telemeter (B. J. A. p. 724, 1908). The image seen in the finders should correspond exactly with that on the plate. When the rising front is used special arrangements have to be made to ensure the correspondence of the images in the finder and on the groundglass. This is done in the " Adams Identoscope " (1908), which is fitted to the swing front and adjusted by a lever to follow the movement of the lens.

The dark-slides or backs, holding sensitive plates, are made either single or double, the former usually for wet plates, the latter for dry plates. The ordinary book-form double dark-slide has been in use since the early days of calotype paper negatives, and contains two plates separated by a blackened metal plate; three of them usually form a set, the shutters being numbered 1 to 6, the odd numbers on the opening side. Inner frames can be used for smaller plates if desired. The slides should fit easily into the camera and the shutters run smoothly out and in. They must be perfectly light-tight, the corner joints, the hinges in the shutters, and the openings in the sides and top of the bookform slides are all weak points requiring occasional careful examination or protection by metal plates. The shutters of dark-slides are either jointed or solid and removable; the former is perhaps the more convenient, but both forms may become liable to let in light. Various forms of solid slides, single and double, are now made in wood or metal, or of wood for the frame and metal for the shutters; they are lighter, more compact and less liable to admit light to the plates. In some cases one slide can suffice for the exposure of several plates or stiff films, enclosed in separate envelopes, as in the " Wishart-Mackenzie " slide, the " Victrix " and other similar ones, or contained in a single packet, as in the " Premo Filmpack," and similar arrangements which enable twelve thin celluloid films to be placed in the camera, exposed one after the other, and removed again safely in daylight, the pack being replaced, if necessary, by another. The packets of films are made of light cardboard, and effect a great saving of bulk and weight (fig. 1). Rollholders are also a convenient way of carrying sensitive celluloid films in lengths of six or twelve exposures, rolled on spools, which can be changed in daylight. Changing boxes for holding a reserve of plates or celluloid films in sheaths, are used with some magazine and other cameras. They are arranged to fit on the camera in place of the dark-slide and the plates are changed automatically so that exposed plates are placed in order successively at the back, a fresh plate going forward for exposure and the number of the exposure being recorded at the same time.

Studio cameras, for portraiture, are usually of the square bellows type, of solid construction, to take large and heavy lenses; adjustable from front and back with rack and pinion movements, to enable long or short focus lenses to be used, with extra extension for copying or enlarging. They are generally fitted with repeating backs, allowing two or more exposures to be made on one plate. The backs are square or reversible, so that the plates can be used upright or lengthways, and are fitted with double swing movements at the back. When single dark slides are used they are best fitted with a flexible shutter to avoid jerking and movement of the camera. For portraiture they are mounted on solid pillar stands, being raised or lowered with an endless screw or rack-work, and the table-top usually has vertical and horizontal angular movements. Large cameras with long extension for copying purposes are made in many forms with special arrangements for the various photo-mechanical processes, and are mounted on substantial table-stands with screw adjustments for obtaining the various motions above noted, and also a rectilinear traversing motion right or left. All these stands should be absolutely rigid and free from tremor. Process cameras are, however, sometimes mounted, FIG. i. - Premo Film-pack.

together with the copying board, on swinging stands, to avoid the effects of vibration.

Portable and field cameras include cameras of the Hare and Meagher types for outdoor work and general purposes on plates 15 in. X 12 in. to 81 in. X 62 in., and in lighter forms from 62 in. X 44 in. to 44 in. X 34 in. For general purposes they are usually made with square bellows and folding tail-board, rather more substantially than those with conical bellows intended for outdoor work. There are many patterns, the principal modern improvements in field cameras being swinging fronts, tripod head and turntable in the base-board, double and sometimes triple extension movements from the back and front for long or short focus lenses, and the use of aluminium for some of the metal-work. They are fitted with a focusing screen and are intended for use on a tripod stand, though some of the smaller sizes of the modern light hand or stand cameras can be used as hand cameras with finders. The plates are carried in the usual dark-slides, but the smaller sizes, from half-plate downwards, can be fitted with roll-holders for flexible films, or with film packs or other daylight changing arrangements.

Folding and Hand Cameras. - Folding cameras form a class of modern portable cameras which have many conveniences for hand or stand work from quarter-plate to 7 in. X 5 in. They may have all the fittings of a stand camera and be made to take glass plates, flat or roll films, but have the advantage of forming when closed a convenient package encolsing camera, lens and shutter, all in position for immediate use when opened out (fig. 2). Most of them are fitted with focusing glass and finders, and may focus by scale in the same way as hand cameras. With an apparatus of this kind on a light stand any class of ordinary indoor or outdoor work can be undertaken within the size of the plate, and the extension of the bellows, which should be quite double the focus of the lens.

The multiplicity of forms and arrangements of hand cameras makes it difficult to classify them into distinct types; but they may be mainly divided into box and folding cameras, and further into (a) cameras with enclosed changing magazines for plates or flat films; (b) with enclosed roll film on spools; (c) with separate changing magazines, changing boxes or roll-holders; (d) with single, double or multiple plate carriers or film-packs. Most cameras that will take glass plates in the ordinary plate-holders will take cut films in suitable sheaths or can be fitted with envelope slides, film-packs or roll-holders. The normal size for hand cameras is the quarterplate (44 in. X 34 in.), or the continental size 9 X 12 cm.; 5 in. -}- 4 in. is also a popular size, and cameras for the post-card size, 51 in. X 31 in. or 15 X to cm. have been largely adopted. Smaller sizes are also made for lantern plates and for the lighter pocket cameras, some in the form of stereoscopes, field-glasses or watches, as in the " Ticka," but the pictures are small and require enlarging. Hand cameras are constructed on the same principles as stand cameras, but, being specially intended for instantaneous work, they are simplified and adapted for rapid focusing and exposing. The focusing screen is superseded or supplemented by finders arranged to show the limits of the subject on the plate, the focus being adjusted by the infinity catches and focusing scales above noticed. Swing-backs and fronts are often dispensed with, but are desirable adjuncts, and a rising and falling front particularly so. Lenses of fairly large aperture, f/6 to f/8, and good covering power, preferably of the anastigmatic type, or a rapid aplanat, should be used, but for very rapid work anastigmats working from f/4 to f/6 will be more useful. Hand cameras can also be fitted with telephoto objectives of large aperture. Some cheap hand cameras are fitted with single landscape lenses or aplanats working about f /11 or lower, but the want of intensity limits their use to well-illuminated subjects. Shutters of the between-lens type are now generally used in hand cameras, and for ordinary purposes should give fairly accurate exposures from k to 5 1 0 of a second or less and also time exposures. Some central shutters are speeded for shorter exposures to s o of a second, but for these focal plane shutters are preferable, and for the more rapid exposures to inn) of a second and less are necessary. The shutter should be efficient, regular in action, and readily released by gentle pressure, pneumatic or otherwise. Mechanism for automatically changing plates or films in hand cameras of the box magazine type must be certain in action, simple and not readily put out of order, special care being taken to avoid rubbing or abrasion of the plates in changing or transport. In changing plates or films the number of plates exposed should be recorded automatically, and duplicate exposures prevented as far as practicable. A circular level placed near the finder is useful.

The choice of a hand camera depends upon the circumstances in which it is to be used, and the purpose for which it is principally required. Fbr general work and with the modern facilities for carrying and changing plates and films in daylight, the numerous folding hand ar stand cameras for plates, flat or roll films, with full adjustments, will be found most useful. Box or magazine cameras in which a supply of cut films or plates can be carried, changed mechanically, and exposed rapidly in succession, are convenient, but their use is limited and they are liable to get out of order. A third class are the reflex and other hand cameras with focal plane shutters for specially rapid instantaneous work as noticed below. There are two types of light folding hand or stand cameras, specially adapted for hand camera work - those made for taking glass plates and cut films, and the folding pocket Kodak or other roll - film cameras. The former are now made of very light construction with mahogany or metal bodies, wooden or aluminium baseboards, thin metal darkslides (fig. 3). The cameras of the pocket Kodak type are of similar construction, but made to take roll films on spools, or with an attach ment for focusing glass and dark-slides for taking plates and cut films. Attached to a slingstrap the quarter-plate size can be: quite conveniently carried in a side-pocket. Watson's " Deft " folding camera is fitted with a focal plane shutter (fig. 4). The " Selfix carbine " camera has a self-erecting front bringing the lens at once into position for use on opening out. Those fitted with lenses of fairly large aperture, double extension, and rising and falling fronts are to be preferred. Of box or magazine cameras there is an immense variety. In some the lens is fixed in focus for all objects within a certain distance, in others it is adjusted by a focusing scale on the lens or by an extending front. Some have a single magazine, others two or more. Some take only glass plates, others plates or cut films. All of them are, however, self-contained and ready for immediate exposure. One of the earliest forms of single magazine cameras, still in use, as in the " Eureka " and " Yale," is the " bag," in FIG. 5. - Double-magazine Box FIG. 6. - The Verascope, Camera. Richard.

which a supply of plates or films in sheaths, is kept in a magazine behind the camera, ready for exposure, the plates as exposed being lifted with the fingers into a bag or expanding chamber above the magazine and placed behind the rest of the plates at the back, a fresh plate taking its place in front. In some forms the magazines are removable and replaceable by others. The arrangement is simple and effective, but the bag, usually made of soft leather or cloth, is liable to wear and puncture, and may make dust. The cameras with double magazines in which unexposed plates are kept in one recess and transferred successively after exposure to a second recess are more complicated, and many FIG. 2. - Sinclair Folding Camera.

FIG. 3. - Ernemann's Pocket Camera.

FIG. 4. - The " Deft " Folding Focal-plane Camera.

ingenious devices have been invented for effecting the change (fig. 5). Some forms are effective and popular on account of their compactness and readiness for immediate exposure, but there is always a risk of the mechanism failing, and care has to be taken in charging them to lay the plates truly in their places. The very handy binocular cameras, or photo-jumelles, of which the " Verascope " (fig. 6) is a type, are of this class, and have additional FIG. 7. - Beck's Dai-Cornex Daylight-loading Camera. magazines. So also are hand cameras of R. and J. Beck's " Frena " type, specially constructed for using stiff celluloid films. The films are notched on two sides and packed in bundles alternately with cards similarly notched. The pack of films and cards is placed in a magazine at the back of the camera, and by the movement of a lever, after exposure, the exposed film and its following card are released, and by turning the camera down are dropped into a second receptacle. A " folding Frena " is now made as a folding camera with attached magazine for films, without which it can be used separately for plates. R. and J. Beck's new " Dai-Cornex " is a great improvement in this form of camera, being a daylightloading box magazine camera for plates, the plates being packed in a bundle of ridged sheaths, so that they are FIG. 8. - Watson's " Vril " Camera. quite protected from light and can be put into or taken out of the camera in full daylight. In other respects it resembles other magazine cameras (fig. 7). Another useful magazine camera is the " Zambex," carrying either plates or films, held in skeleton frames in envelopes which can be loaded or unloaded in daylight, and are kept ready for use in the back of the camera and exposed consecutively. For work in which speed is of primary importance hand cameras fitted with very rapid lenses and focal plane shutters are necessary, and several forms of portable collapsible cameras of this kind are now available, such as the Goerz-Anschiitz, Zeiss's " Palmos," Watson's " Vril " (fig. 8), Adams " Idento," &c., and are lighter and more portable than the reflex cameras. Hand cameras are generally fitted with screw-bushes for mounting on a tripod stand when time exposures are wanted. The light folding e wooden or aluminium stands noted below are specially suitable.

For photographing animals, objects in motion, public functions, &c., it is important to have the means of watching the movement till the critical moment of exposure arrives. For this it is convenient to have a camera fitted with twin lenses working in two separate compartments (fig. 9) or more simply with a mirror throwing a full-sized unreversed image of the object from the lens on to the focusing screen (fig. to). With the former, which has the advantage that the image is seen before, during, and after exposure, the lenses must be of exactly equal focus and focused together by the same motion of the rack-work, the object being viewed on the focusing screen of the upper compartment, and the plate kept ready in the lower to be exposed when desired. Binocular hand cameras are also made on this principle, one compartment serving for focusing, the other holding lens and plates. Stereoscopic cameras are another form of twin-lens cameras, and are usually made for also taking single panoramic pictures.

In reflex cameras only one lens is necessary, though two are convenient, and can be used somewhat as in fig. 9. They generally consist of a cubical box camera containing a movable mirror facing the lens at an angle of 45° and throwing up the image projected from it on to a horizontal focusing screen, on which it is viewed through a flexible hood which folds down in the upper part of the camera when not in use (fig. 10). In order to get the greatest rapidity of exposure a focal-plane shutter is generally fitted, and by a single movement of the release the mirror is smoothly lifted and the plate exposed simultaneously. They should be fitted with anastigmatic lenses working at large apertures for very rapid work. In some forms the lens is fixed, but usually there is a front bellows extension for long-focus lenses, with rising and falling front, to which swing motion may be given, a swing-back not being generally used with the focal plane shutter. In the " Ernex " camera E. Human has made an arrangement by which the camera back, horizontal viewing screen and reflector are made to swing simultaneously, by a rack and pinion movement. They may also have reversing or revolving backs for quickly changing the position of the plate. 5 in. X 4 in. and 34 in. X 44 in. are the usual sizes of the plates, but larger and smaller sizes are also available. These cameras require the best workmanship and perfect mechanism for successful working and freedom from any jarring movement in releasing the shutter or mirror. The focusing screen must also be in accurate register with the focus of the lens on the plate. Those forms in which the image can also be viewed at the height of the eye, as in the Graflex (fig. to), are preferable. Although reflex cameras are rather heavy and bulky as hand cameras, they have many advantages over the ordinary hand camera with finder and focusing scales for the purpose of the press photographer, the naturalist and others, in observing and recording very rapid movements, and have come into very general use for such purposes. They permit the accurate focusing of a fullsized image on the groundglass up to the moment of exposure, especially useful when lenses of long or short focus are required and when the rising or swing front is in use. The aspect of this image on the ground-glass is also a great aid in the selection and placing of the subject and in judging the exposure required for it. They practically have all the advantages of a stand camera and can be used as such on a stand for subjects requiring prolonged exposure. They are also coming into increasing use in studio work for portraits of children, &c. Their use and adjustments are discussed by G. E. Brown in the British Journal Almanac for 1909.

Many so-called " panoramic " cameras have been introduced from time to time, among them T. Sutton's (1861), and J. R. Johnson's " Pantascopic " (1864), but did not FIG. 1 1. - Section of " Al-Vista " Panoramic Camera.

come into general use till the use of curved surfaces of celluloid film enabled such cameras of convenient size and weight to be put on the market. They are on the same principle as one made by F. von Martens in 1845 for curved daguerreotype plates, and covering an angle of 150°. P. Moêssard's " Cylindrographe " of 1889 was the first of the modern type. It consists of a semi-circular FIG. 9. - Camera fitted with Twin Lenses, section to show working.

A, Hood of finder.

B, Ground glass screen. c, Mirror.

D, Viewing lens.

E, Working lens.

F, Shutter.

G, Focusing pinion.

II, Plate carrier.

I, Plate.

FIG. Io. - Reflex Camera A. Lens.

B. Mirror.

C. Ground-glass.

D. Plate.

E. Supplementary mirror.

camera, the front of it formed of light-proof cloth and the back by the curved flexible carriers. The lens is fitted on a vertical axis, so that the nodal point of emergence remains motionless, and is revolved round it by means of a handle worked by hand and carrying a view meter. The illumination of the image is regulated by an adjustable vertical slit in a tube attached to the lens inside the box, and by altering the rate at which the lens is revolved. The pictures taken embrace less than 180°. The apparatus folds together and is quite portable; it is fully described in Moessard's Le Cylindrographe (Paris, 1889). The " Al-Vista " (1901) and the "Panoram Kodak " (1900) are on the same principle, but arranged as roll-holder hand cameras, in two sizes, carrying film for several exposures, 7 in. X 24 in. or 4 in. X 12 in. They work instantaneously, and by means of a clock-spring the lens rotates rapidly over a half-circle when released. The angle of view is about 120° (figs. 11 and 12). The views taken with this kind of camera are sometimes disappointing, on account of the development of cylindrical perspective on a plane surface causing apparent distortion. This distortion is avoided in Carl Zeiss's " Palmos Panoram " camera for plates 64 in. X 3± in., fitted with " Tessar " lens and focal plane shutter, and other similar cameras which can be used for stereoscopic or single pictures. Other more elaborate instruments driven by clockwork have been made for making a complete tour of the horizon. Among them C. Damoizeau's " Cyclographe," which can be used with lenses of different foci and takes the pictures on a roll-film, which is unrolled as the instrument revolves on its axis, the lens also rotating on its nodal point of emergence; and thus the image always remains sharp (Bull. Soc. Franc. d. Phot., 1891, p. 183,). Commandant A. Daubresse has improved on Moessard's apparatus, by placing the lens vertically between two right-angled prisms, the upper of which receives the image and projects it through the lens on to the lower prism, from which, by rotation of the system on the vertical axis, it is projected on to a cylindrical film through an angle of 360° (Ibid. 1906, p. 430; E. Jb., 1907, p. 91). The " Periphote " and Ernemann's " Rundblick " camera are improved forms (E. Jb., 1908, p. 322).

Many early forms of panoramic cameras are described in B. J. A. 1892, p. 517. Colonel R. W. Stewart's " Panoram " (1893), A. Chevalier's " Photographic Plane Table," J. Bridges Lee's " Photo-Theodolite " (1894), and similar cameras fitted with telescopes, levels and divided circles, are instruments of precision suitable for photographic surveying. Improved instruments for topographical surveying with stereo-photographic apparatus, on the principle worked out by Dr C. Pulfrich, of Messrs Zeiss & Co., in his stereo-comparator (1903), are being practically developed, and much information regarding them will be found in papers by E. Dolezal and others in J. M. Eder's Jahrbiicher, 1903 to 1908; also a paper by Lieut. F. V. Thompson in Geographical Journal, 1908, xxxi. 534.

Many forms of camera have been constructed for making the three negatives required for trichromatic photography. They fall into two types: (I) those with a repeating back fitted with three colour-screens or filters - red, green.

and violet - through which the colour impressions are made suc cessively with one lens upon a single colour-sensitive plate, as in the Sanger-Shepherd system. The colour-screens are placed immedi ately in front of the sensitive plate in the repeating back, which is moved on for each exposure. In a more recent form, by the same FIG. 13. - Diagram of Camera maker, the three images are taken for Three-colour Photography. on the sensitive plate with one exposure. The camera is divided into three compartments, and fitted with a special diaphragm which can be regulated for the varying sensitiveness of different batches of plates. The central image is impressed directly on the plate; the other two by reflection from prisms arranged so as to equalize the sizes of the three images on the sensitive plates, the light rays passing in each case through a suitable colour-filter - red, green and blueviolet - somewhat on the principle of F. E. Ives's camera of 1900 (fig. 13). It is convenient and successful in working. (2) Cameras made on the reflecting principle of L. Ducos du Hauron (1876), elaborated by F. E. Ives (1894) in his photochromoscope, in which three images are taken through three colour-screens on separate plates with one lens, the respective exposures being regulated by reflection of the light coming from the lens by plane mirrors on to the sensitive plates, and its filtration through the colour-screens in front of them. Many variations of this method have been proposed, in which reflecting prisms replace the mirrors. The different systems have been discussed by W. Gamble (Ph. Jour. 1905, xlv. 150), the latter also by E. T. Butler (Ibid. p. 199). Sir W. de W. Abney has described three-colour cameras for landscape work in Ph. Jour. 1904, xliv. 81, and 1908, xlviii. 331.

Enlarging Cameras. - These cameras vary in form, according to the nature of the illumination, but ordinarily consist of a double or triple extension bellows camera, with a holder for the negative or transparency at one end, and for the sensitive plate or paper at the other, the lens being placed on a fixed partition between the two. Some recent forms of " daylight enlargers " can be used as an ordinary camera. Other cheaper ones are on the fixed focus principle. Enlargers for use with artificial light are made like a magic lantern, with a condenser, projecting an enlarged image on to a sensitive plate or paper fixed on an easel or screen. A simple arrangement for daylight enlarging is to fix a suitable camera on to a larger one by a sliding front, and mount the two on a studio stand tilted so that the image may be illuminated by the open sky.

Many special cameras and lenses have been introduced for taking on a long flexible sensitive film an extended series of small photographs of the successive phases of movements, and again projecting them on a screen so as to reproduce the scene, with an illusion of motion, in what are known as " living pictures," biographs, &c. As each photograph requires a certain minimum time for exposure and must be kept in true position in sequence with the rest, some means of regulating the intermittent exposures and keeping the film in position have to be adopted; and there are many different ways of doing it, either by a continuous or intermittent motion and exposure of the film while it is being unwound from one roller on to another. The films used are similar to the ordinary celluloid films, but in narrow bands from 18 in. to 21 in. in width, the length varying with the number of exposures required, at the rate of 16 to 20 per second. They are perforated on both sides, so that they may run true and have the necessary intermittent motion, the perforations fitting on to studs on a sprocket wheel in connexion with the driving wheel and crank handle. Special lenses of short focus, from I in. to 3 in., with good covering power and large apertures f/4 to f/2, are required both for photographing and projecting; several such are noted below. Absolute rigidity in the camera is essential. Special stands are made for the purpose, but if a tripod stand is used it should be well braced. Special apparatus is required for developing and fixing the exposed films. They are wound on large rollers supported over troughs containing the necessary solutions (see Cinematograph). The mechanical arrangements are treated in H. V. Hopwood, Living Pictures (1899); F. P. Liesegang, Handbuch der praktischen Kinematographie (1907); K. W. Wolf-Czapek, Die Kinematographie (1908); G. Lindsay Johnson, Photographic Optics (1909); Eder's Jahrbiicher. A method of cinematography in colour was introduced by G. A. Smith and C. Urban in 1908, the main features of it being the use of a film sensitive to all colour waves to the furthest red; superimposing the colour records by persistence of vision; the use of two-colour records instead of three, in order to reduce the interval between the successive presentations; adaptation to existing cinematograph machinery and films. These conditions are fulfilled by the use, in place of the ordinary revolving sector shutter in front of the lens passing intermittent white light, of a special, more rapidly revolving shutter divided into four sectors, one fitted with orange-red glass, another with bluish-green glass and two intermediate opaque sectors, so that at every revolution of the shutter an exposure is made through the red and green glasses alternately. The former passes white and yellow, and then orange, scarlet to deepest red; whilst the latter also passes white and yellow, green, blue-green, blue, all in proportion according to the red and green sensitiveness of the specially sensitized panchromatic emulsion on the film. The same shutter and colour screens are used for projection, some supplementary blue rays being added. The results are satisfactory and the method promises to be of great practical value (see Jour. Roy. Soc. Arts, 1908, 57, No. 2926).

Special cameras are made for various branches of scientific research in photo-micrography, photo-spectroscopy, astronomical photography, &c.

Field cameras are usually supported on wooden tripod stands, folding in two or more sections, the head being separate or fixed in the base-board of the camera. The legs should be capable of extension to about 5 ft. and adjustable in length for use on uneven ground. A tripod stand may be light, but must be firm and rigid when set up. To prevent slipping, shoes of indiarubber or cork may be fitted to the points of the legs, and in some cases it may be desirable to strengthen the tripod by a folding adjustable brace. W. Butler's " Swincam " camera stand is made to enable the camera to be securely fixed in awkward positions, and has many valuable special features, great extension, swivel points to the feet, &c. For hand cameras the very light, portable metal folding and walking-stick stands are convenient.

FIG. 12.-" Al-Vista " Panoramic Camera, closed.

Photographic Objectives or Lenses. The objective is the most important item of photographic apparatus, because upon it depends the perfection with which a correct and well-defined picture is projected upon the plane surface of the sensitive plate of objects in the different planes forming the field of view, which naturally would come to a focus on a series of curved surfaces. This flattened picture must be equally illuminated and sharply defined, within a limit of confusion from l b to 7 16 of an inch, over a sufficiently wide angle. A good objective must also pass sufficient light to produce the required effect on the photographic plate with short exposures; the chemical and visual foci must coincide exactly, and it must not distort straight or parallel lines. The fulfilment of these conditions is complicated by the presence of sundry focal displacements or aberrations. (I) Spherical aberration, or non-coincidence of the foci of the central and marginal pencils of rays passing through the lens. It is corrected by varying the curves of the component lenses and by the use of a diaphragm. (2) Coma, or blur, due to lateral spherical aberration of oblique rays, and mostly found in unsymmetrical combinations and single view lenses. It is partly eliminated by the diaphragm. (3) Astigmatism, which accompanies coma in single lenses, and is usually present in symmetrical aplanats, manifests itself by forming two sets of images of points off the axis, lying in two separate curved surfaces, one set focusing tangentially as more or less horizontal lines, the other radially as more or less vertical lines. It increases with the obliquity of the rays and causes want of definition and difference of focus between horizontal and vertical lines away from the centre. (4) Curvature of field, also increasing with the obliquity of the rays. (5) Distortion, outward or inward, according to the nature and construction of the objective. With the single meniscus view lens, used with its concave surface towards the object and a diaphragm in front, a square will appear barrel shaped from inward contraction of the lines towards the centre; but with the convex surface towards the object and the diaphragm behind, it will appear with concave sides from outward expansion from the centre. It can be corrected by using two such lenses with the convex sides outwards and a central diaphragm, as in periscopic or rectilinear lenses. Lenses of the orthoscopic and telephoto types generally show the latter form of distortion. (6) Chromatic aberration, produced by the dispersion of the white light passing through the lens, and the different coloured rays composing it coming to a focus at different distances from the visual focus in the order of their wave-lengths. It thus affects both the positions and sizes of the image for the different colours. For ordinary photographic work it suffices for the blue-violet and yellow rays to be coincident, but for the new processes of photography in three colours, apochromatic lenses, in which perfect coincidence of the coloured rays is secured, are required to obtain the accurate register of the three images. The corrections are effected by compensating lenses of different refractive powers (see Aberration).

In constructing photographic objectives these aberrations and distortions have to be neutralized, by regulating the curves of the different positive and negative component lenses, the refractive and dispersive indices of the glasses from which they are made, and the distances of the refracting surfaces, so as to make the objective as far as possible stigmatic or focusing to a point, giving an image well defined and undistorted. This perfect correction could never be effected in objectives made before 1887, and very few could be effectively used at their full apertures, because although linear distortion could be overcome there were always residual aberrations affecting the oblique rays and necessitating the use of a diaphragm, which by lengthening out the rays caused them to define clearly over a larger surface, at the expense of luminous intensity and rapidity of working. The introduction of rapid gelatin dry plates enabled photographs to be taken with much greater rapidity than before, and led to a demand for greater intensity of illumination and better definition in lenses to meet the requirements of the necessarily very rapid exposures in hand cameras. For studio and copying work quick-acting lenses are also valuable in dull weather or in winter.

The rapidity of a lens with a light of given intensity depends upon the diameter of its aperture, or that of the diaphragm used, relatively to the focal length. In order, therefore, to obtain increased rapidity combined with perfect definition, some means had to be found of constructing photographic objectives with larger effective apertures. This necessity had long been recognized and met by many of the best makers for objectives of the single meniscus and aplanatic types, but with only partial success, because such objectives are dependent upon the diaphragm for the further correction necessary to obtain good definition over an extended field. The difficulty was in the removal of astigmatism and curvature of the field, which, as J. Petzval had shown, was impossible with the old optical flint and crown glasses. In 1886 Messrs E. Abbe and O. Schott, of Jena, introduced several new varieties of optical glasses, among them new crown glasses which, with a lower dispersion than flint glass, have a higher instead of a lower refractive power. It was thus rendered possible to overcome the old difficulties and to revolutionize photographic optics by enabling objectives to be made free from astigmatism, working at their full apertures with great flatness of field independently of the diaphragm, which is now chiefly used to extend the area of definition or angle of view, and the so-called " depth of focus " for objects in different planes.

Photographic objectives may be classed as follows: I. Single achromatic combinations.

2. Unsymmetrical doublets.

3. Symmetrical doublets.

4. Triple combinations.

5. Anastigmatic combinations - symmetrical and unsymmetrical.

6. Telephotographic objectives.

7. Anachromatic combinations.

They are also sometimes classified according to their rapidity, as expressed by their effective apertures, into " extra rapid," with apertures larger than f/6; " rapid," with apertures from f/6 to f/8; " slow," with apertures less than f/I t. Another classification is according to the angle of view, " narrow angle " up to 35°; " medium angle " up to 60°; " wide angle " up to 90°, too°, or more. Many lenses are made in series, differing in rapidity and angle of view as well as in length of focus.

t. Single Achromatic Combination or Landscape Lens. - This is the earliest form of photographic objective, evolved from W. H. Wollaston's improved single periscopic meniscus camera obscura lens (1812). It was made achromatic by Ch. Chevalier, and so used by L. J. M. Daguerre, though it required correction for chemical focus, as did the object glasses of telescopes or opera glasses first used for photography. The single landscape lens usually consists of an achromatic compound meniscus, formed of a biconvex positive crown cemented to a biconcave negative flint to secure achromatism and partially correct the spherical aberration, and may be taken as the type of the " old photographic achromat " (fig. 14). 1 It is used with its concave side towards the object and a diaphragm in front, thus producing inward or barrel-shaped distortion, inherent in this type of objective, and rendering it unsuitable for copying or architecture, though not very noticeable in landscape work. The full aperture has to be largely reduced by a diaphragm to improve definition; so it is slow, though many improved forms have been brought out. It has always been popular for pure landscape work on account of the equality of illumination over the plate, depth of focus, and the softness and brilliancy of the image owing to its thinness and freedom from reflecting surfaces. In some of its improved and " long focus " forms it is preferred by portraitists for large heads, on account of the general softness it gives when used with large apertures.

The following are some of the best-known improved objectives of this type: T. Grubb's "Aplanatic " (1857), f/15 to f/30: FIG. 15. - Grubb's FIG. 16. - Rapid Landscape Lens.

" Aplanatic " Lens. Long Focus.

(fig. 15); J. H. Dallmeyer's " Wide Angle Landscape Lens " (1865), f/15, angle 75°. In it distortion was reduced and marginal definition improved. The " Rapid (long focus) Landscape Lens " (1884), f/12, angle 40° (fig. 16), was a modification of it, and at f/8 is useful for heads in portraiture. W. Wray's " Landscape Lens " (1886), f/II, is also useful for portraiture in the larger sizes at f/8. Fr. Voigtlnder's " WideAngle Landscape Lens " (1888) In the diagrams of lenses which follow, a uniform system of indicating the nature of the glass employed by means of the shading has been adopted.

Flint glass is indicated thus Crown glass of low refractive power thus: - Crown glass of high refractive power thus: (These two are used indiscriminately in lenses made before the introduction of the new Jena glass.) Extra light flint glass thus In most cases the front of the lens is on the right.

Old types. New types.

FIG. 14.-Single Landscape Lens.

???

f/15, angle 90°, with great covering power and depth of focus. T. R. Dallmeyer's " Rectilinear Landscape Lens " (1888), f/14, angle 60° (fig. 17), was of novel construction, free from distortion, brilliant in working and useful for copying. Messrs Ross's " WideAngle Landscape Lens " (1890), f/16, angle 70°, triple cemented and made of Jena glass. Many other excellent objectives of this type have been made by British and foreign makers and are still used, though somewhat superseded by the fully corrected anastigmats specally made to work singly, or as single elements of anastigmatic doublets, as noticed in § 5.

2. Unsymmetrical Doublets: Old Types. - This class includes objectives with comparatively large apertures formed of two dissimilar combinations, in most cases correcting each other, with a diaphragm between them. In some the single elements may be used independently. All the older " portrait " lenses, some of the aplanatic doublets and Fr. von Voigtlander's " Orthoscopic " Lens (1857), now disused, are of this type. Even with the present improved conditions, the portraitist working in a studio requires a quick-acting objective of large effective aperture and comparatively short focus, giving a brilliant well-defined image of near objects in different planes over a restricted field of view. The early single lenses were found to be too slow for portraiture by the daguerreotype and talbotype processes, and the efforts of opticians were directed to the problem of obtaining the maximum amount of light, together with good definition and flatness of field, and about 1840 compound lenses were brought out by Andrew Ross and C. Chevalier, consisting of two achromatic compounds, one at each end of a tube. Ross's lens, made for H. Collen, is interesting as the first lens corrected photographically, so that the visual and chemical foci were coincident (fig. 18). Ch. Chevalier also combined lenses of different foci, as is now done for " convertible " objectives, used singly or combined. He also fitted them with iris diaphragms. These forms were soon superseded by the compound portrait lens, calculated by J. Petzval and brought out by FIG. 18. - First English Portrait Lens. FIG. 19. - Portrait Lens.

Fr. von Voigtlander in 1841. It consists of two dissimilar achromatic combinations widely separated. At first the diaphragms were in front, but now they are central. The front element is a plano-convex composed of a biconvex crown cemented to a planoconcave flint, while the back element is a double convex, composed of a biconvex crown separated by an air-space from a concavoconvex flint (fig. 19). This form of objective quickly supplanted all other for portraitures, and is still largely used, though it has defects which prevent its use for general purposes and is being superseded for portraiture by some of the rapid anastigmats. In his " Quicl. Acting Portrait Lens " (1860), f/4, angle 45, °, J. H. Dallmeyer improved the correction for spherical aberration, and in his " Extra Quick Acting Portrait Lens " (1860), f/2.2, used for cinematograph work, attained greater rapidity. In the " Patent Portrait Lens " (1866), f/3, f/4 and f/6, angles 50 0 to 55° (fig. 20), he made great changes in the form and relative positions of the back elements, giving a flatter field and freedom from flare spot. By separating the two components of the back element more or less spherical aberration could be introduced to give softer definition 20. - Dallmeyer's Patent and greater depth of focus. In Portrait Lens. 1875 Dr. A. Steinheil made an unsymmetrical aplanatic por trait combination of peculiar construction, working at f/3.2. It was an improvement on his similar symmetrical " Portrait-Aplanat," FIG. 21. - Portrait Antiplanet. FIG. 22. - Group Antiplanet. Form I. of 1874, but was superseded in 1881 by the " Portrait Antiplanet," f/4 and free from astigmatism over an angle of 14°.

It had six reflecting surfaces and nearly approached a triplet (fig. 21). Steinheil's " Group Aplanats " (1879), f/6.4, angle 70°, were an improvement on the ordinary " Aplanats," but were superseded in 1881 by the " Group Antiplanets," f/5, angle 70°, lenses of a distinct type (fig. 22). They were a further advance on the " Aplanats," working at larger apertures and giving better definition. This lens is interesting as the first in which astigmatism was eliminated by combining a " crown-shaped " lens of high refractivity, with a " flint-shaped " of lower refractivity,. though made of the old glasses. In his " Rapid Antiplanet " (1893), f/6.5, angle 30°, Dr R. Steinheil improved the " Group .Antiplanet " as regards astigmatism and covering power by replacing the thick back combination by a triple longfocus negative element consisting of a crown between two flints, with a heavy barium crown in the front element instead of a flint (fig. 23). Voigtlander, who originally constructed the Petzval portrait lens, improved it in 1878 and 1885, and now makes two lenses on the same principle, series I. f/3.2, angle 28°, for ordinary portraiture and projection, and series Ia., f/2.3, angle 22° (190o) for astrophotography, cinematography, &c., when intense illumination is required over a small field. Both are quite free from coma.

FIG. 23. - R. Steinheil's ImFIG. 24. - Ordinary Angle proved Group Antiplanet. Actinic Doublet.

Most of the above are portrait objectives of large aperture, but unsymmetrical doublets have also been made for landscape work. J. T. Goddard's " Combination Landscape Lens " (1859) was one of the first, and was free from distortion, gave a flat field, and could be used as a convertible lens. In 1864 T. Ross issued his " Actinic Doublets," modified from the Collen lens, in three series - " small angle," f/8, angle 40° to 50°; " ordinary angle," f/14, angle 60° to 75° (fig. 24); " large angle," f/16, angle 80° to 95°. These lenses were similar to the " Globe," but unsymmetrical and more rapid. The separate elements could be used alone_ Some of them were fitted with a shutter near the diaphragm. They were superseded by the " Symmetrical " lenses.

3. Symmetrical Doublets. - This class includes objectives formed of two similar combinations of lenses, usually of the convergent meniscus form, with their concave surfaces inwards and a diaphragm between them; consequently they are rectilinear and practically free from marginal distortion. Until the introduction of anastigmatic doublets they were in general use for all purposes under the names " Aplanat," " Rectilinear," " Symmetrical," " Euryscope," &c. They are still largely used and have been improved by the use of Jena glasses in their construction.

The first recorded lens of this type was Dr J. W. Draper's combination used in 1839 for daguerreotype portraits, consisting of t vo double-convex lenses 4 in. diameter, with a united focus of 8 in., mounted in a tube with a diaphragm 32 in. in front. In 1841 T. Davidson made a combination of two single landscape lenses very similar to the later rectilinear doublets. Being slower than the Petzval portrait lens its value as a non-distorting lens for general purposes was not recognized. G. S. Cundell (1844) combined two uncorrected meniscus lenses with a diaphragm between them. In 1860 T. Sutton brought out his " Panoramic Lens," which worked on curved plates covering about loo°. It was followed FIG. 25. - C. A. Steinheil's Periskop." FIG. 26. - A. Steinheil's "Aplanat." by C. C. Harrison's " Globe Lens " (1862), angle 75°, composed of a symmetrical pair of deep compound menisci, the exterior surfaces forming part of a sphere. Though defective and slow it was popular for a time. C. A. Steinheil's " Periskop " (1865, f/13.5, angle 90° was a symmetrical doublet formed of two plain crown menisci with central diaphragm (fig. 25). It gave a larger field than the " Globe," the lenses being closer together. Being nonachromatic it had to be adjusted for chemical focus. It was quite free from distortion, with a very flat field, and both nodal points together. It is considered the best possible combination of two plain lenses, and is still used in some of the cheaper hand cameras with fixed focus, the difference of the chemical and visual foci being allowed for in the camera or by adjustable lens mounts. G. Rodenstock's " Bistigmats " are of this class. J. Zentmayer made a similar unsymmetrical lens. In A. Steinheil's " Aplanat " (1866) the same principle was carried out with achromatized lenses, and a great FIG.

improvement was effected in the construction of non-distorting objectives of fairly large aperture. It consisted of two positive cemented flint menisci, each composed of a dense flint with negative focus outside and a light flint with positive focus inside, its concave surfaces facing the centre (fig. 26). This use of flint glasses alone was peculiar, former achromatic lenses having been made of flint and crown. These lenses were made in three rapidities: " Ordinary," f/6 or f/7, angle 60°; " Landscape," f/12 to f/15, angle 90°, also used in convertible sets; " Wide Angle Landscape," f/20 to f/25, angle 104°; " Wide Angle Reproduction," similar to the last, but with sharper definition. The "Aplanat " had many advantages over previous doublets and the triplet, being more rapid, perfectly symmetrical, so that there was no necessity for turning them when enlarging, and free from distortion or flare. There was no chemical focus. Each component could be used alone for landscape work with double focus, subject to the ordinary defects of single lenses. By the use of Jena glasses in the " Universal Aplanat " (1886) the components of this lens were brought closer together, its intensity increased, and it was made more portable. J. H. Dallmeyer had been working in the same direction simultaneously with Steinheil, and in 1866 brought out his " Wide Angle Rectilinear," f/15, angle 100°, made of flint and crown, the front element being larger than the back (fig. 27). It was slow for ordinary purposes and was succeeded in 1867 by the well-known " Rapid Rectilinear," f/8, on the same FIG. 27. - Wide-Angle Rectilinear Lens.

principle as Steinheil's " Aplanat, but made of flint and crown (fig. 28). Ross's " Rapid " and " Portable Symmetrical " lenses, Voigtlander's " Euryscopes," and other similar lenses of British and foreign manufacture are of the same type, and still in use. They are excellent for general purposes and copying, but astigmatism is always present, and although they can be used with larger apertures than the triplets they displaced, they require stopping down to secure good marginal definition over the size of plate they are said to cover. By the use of Jena glasses they have been improved to work at larger apertures, and some are made with triple cemented elements.

This class comprises objectives composed of three separate combinations of glasses widely separated from each other. An early form of this type was made by Andrew Ross (1841) for W. H. Fox Talbot, others by F. S. Archer, J. T. Goddard (1859), T. Sutton (1860), but they never came into general use. J. H. Dallmeyer's " Triple Achromatic Lens " (1861), f/Io, angle 60°, now out of date, was an excellent non-distorting lens, very useful for general work and copying (fig. 29). As made by Dallmeyer, the inner surfaces of the front and back components were slightly concave, but in T. Ross's " Actinic Triplets " (1861), f/16, they were flat. The centre lens was an achromatic negative serving to flatten the field.

As already stated, it was found practically impossible to obtain flatness of field, together with freedontfrom astigmatism, in objectives constructed with the old optical glasses. A. Steinheil attempted it in the " Antiplanets," but with only partial success. The Abbe and Schott Jena glasses, issued in 1886, put a new power into the hands of opticians by largely increasing their choice of glasses with different refractive and dispersive powers. Whereas the old glasses had high refractivity with higher dispersion, in the new ones high refractivity with lower dispersion could be set against lower refractivity with higher dispersion.

Between 1887 and 1889 the first attempts to make anastigmatic objectives with the new glasses were made by M. Mittenzwei of Zwickau, R. D. Gray of New Jersey, E. Hartnach and A. Miethe of Berlin (" Pantoscope "), K. Fritsch of Vienna (" Apochromat ") and Fr. von Voigtlander of Brunswick, with more or less success, but progress was hindered by the instability of some of the early glasses, which was afterwards overcome by sandwiching the soft glasses between two hard ones. In 1888 Dr H. L. H. Schroeder worked out for Messrs Ross the " Concentric Lens " (fig. 30) issued in 1892 (Ph. Jour., 16, p. 276). It was a symmetrical doublet of novel construction, each element consisting of a plano-convex crown of high refractivity cemented to a piano-concave flint of lower refractivity, but about equal or higher dispersion. Both the uncemented surfaces were spherical and concentric. At f/16 it gave sharp definition and flatness of field with freedom from astigmatism, distortion or flare over an angle of 75°. It was an excellent lens, though slow, and has been superseded by the " Homocentric " and other more rapid anastigmats. Dr Paul Rudolph, of Messrs Carl Zeiss & Co., Jena, worked out in 1889 a new and successful method of constructing a photographic objective by which astigmatism of the oblique rays and the want of marginal definition due to it could be FIG.'31. - Anastigmat. FIG. 32. - Anastigmat.

Series II. f/6.3. Series IIIa. f/9.

eliminated without loss of rapidity, so that a comparatively extended field could be covered with a large aperture. This he did on the principle of the opposite or opposed gradation of the refractive indices in the front and back lenses, by a combination of two dissimilar systems of single lenses cemented together, the positive element of each having in one case a higher and in the other a lower refractive index than that of the negative element with which it was associated. The front system, relied upon for the correction of spherical aberration, was made of the old glasses, a crown positive of low and a flint negative of high refractivity, whilst the back system, relied upon for the anastigmatic flattening of the field, was made of the new glasses, a crown positive of high and a flint negative of low refractivity. Both systems being spherically and chromatically corrected for a large aperture, the field was flattened, the astigmatism of the one being corrected by the opposite astigmatism of the other, without destroying the flatness of the field over a large angle (see E. Tb., 1891 and 1893; M. von Rohr's Geschichte, and O. Lummer, Photographic Optics, for further details). They were issued by Messrs Zeiss and their licencees (in England, Messrs Ross), in 1890, in two different types. The more rapid had five lenses (fig. 31), two of ordinary glasses in the front normal achromat, and three in the back abnormal achromat, two crowns of very high refractive power, with a negative flint of very low refractive power between them.

FIG. 33. - Anastigmat. FIG. 34. - Satz Anastigmat.

Series VI. Series VIa.

The fifth lens assisted in removing spherical aberrations of higher orders with large apertures. The second type, series IIIa., f/9, 18 99 (fig. 32), had only two lenses, the functions of which were as above. These combinations could not be used separately as single lenses. They are now issued as " Protars," series I Ia., f/8; I IIa., f/9; V., f/18. In 1891 Dr Rudolph devoted himself to perfecting the single landscape lens, and constructed on the same principle a single combination of three lenses, the central one having a refractive index between the indices of the two others, and one of its cemented surfaces diverging, while the other was converging. At f/14.5 this lens gave an anastigmatically flat image with freedom from spherical aberration on or off the axis. It was, however, not brought out till 1893, as a convertible lens or " Satz-Anastigmat," series VI., f/14.5, and VIa., f/7.7 (figs. 33 and 34). In the meantime Dr E. von Hoegh (C. B. Goerz) and Dr A. Steinheil had also been working at the problem and had independently calculated lenses similar to Rudolph's, but, whereas he had devoted himself to perfecting the single lens, they sought more perfect correction by combining two single anastigmatic lenses. to form a doublet. Dr Rudolph had had the same idea, but Messrs Goerz secured the priority of patent in 1892, and in 1893 brought out their " Double Anastigmat," now known as FIG. 35.

Ross-Goerz " Dagor." Series III. Ross-Goerz. Series IV.

" Dagor." It was the first symmetrical anastigmat which combined freedom from astigmatism with flatness of field and great covering power at the large aperture of f/7.7 (fig. 35). Both these types of Zeiss's " Protars " and Goerz's " Dagor " anastigmats have since FIG. 28. - Rapid Rectilinear Lens.

?

FIG. 29. - Triple Achromatic Lens.

FIG. 30. - Concentric Lens.

been made by Messrs Ross in England. Messrs Steinheil brought out their first " Orthostigmats " in 1893, but, owing to patent difficulties, were unable to manufacture them in Germany, and they were issued later in France and England. They were followed by a second type, which has since been issued in several series by Messrs Steinheil and by Messrs Beck in England (fig. 36). According to Dr R. Steinheil (E. Jb., 1897, p. 172) this lens was an application of two principles recognized by Dr A. Steinheil as necessary for the spherical and anastigmatic correction of a lens. He attempted to carry them out in the " Antiplanet," but was prevented by the want of suitable glass. He found that for anastigmatic correction an objective should have the separating surface between two media concave towards the medium of higher refraction (new achromat), and for FIG. 36. - Steinheil's FIG. 37. - " Collinear." " Orthostigmat." Series II.

spherical correction the separating surface should be convex towards the higher refracting medium. A fully corrected cemented lens cannot, therefore, be made with less than three glasses, but with uncemented lenses an air-space may form one of the media. In 1895 Dr D. Kaempfer worked out the " Collinear " for Messrs Voigtlander, constructed on the same principles as the " Orthostigmat," type II., and similar to it (fig. 37). It is made in three series: II., f/5.4 and f/6.3; III., f/6.8 and f/77 (convertible); IV., f/12.5, and the apochromatic collinear f/8, calculated by Dr H. Harting for three-colour reproduction, &c. (Ph. Jour., 1901, 25, P. 323.) In 1894 Dr Rudolph extended the application of his principle by combining the old achromat and the new achromat into a single quadruple cemented lens (fig. 38), which, according to T. R. Dallmeyer, was the most perfectly corrected single lens that had been FIG. 38.

Series VII. f /12.5. Series VIIa. f/6.3. evolved up to 1900, Dr Rudolph having succeeded in obtaining freedom from spherical aberration and astigmatism, and also in eliminating coma (Ph. Jour. 1901, 25, p. 68). These lenses were issued in 1895 as series VII. singly and VIIa., in combinations now known as " Convertible Protars," and the earlier series VI. and VIa. were withdrawn. The single lenses of series VII., f/12.5, angle 85°, have great anastigmatic flatness of field and only very slight marginal distortion, a condition not realized before in a single lens. The relative rapidities of the double combinations of series VIIa. vary from f/6.3 to f/8, according to the lenses used. They are excellent lenses for all general purposes.

In their " Convertible Protars," series IV. (1908), f/12.5, angle 60°, Messrs Zeiss have simplified and cheapened the construction of these lenses by the use of new Jena glasses, so that they consist of three instead of four lens elements cemented together, while possessing the same high efficiency as series VII. They are issued as " single " or " double " Protars, f/12.5 and f/6'3 or f17, also in sets of three or four objectives of different foci, which are combined to give pictures of different angles of view from the same standpoint. With both series when using the " Protar " lens singly, it should be screwed behind the iris diaphragm of the mount, to avoid curvature of the field, and when two such lenses are combined the one with the greater focal length should be placed in front.

In 1895 Messrs Goerz patented a double anastigmat, f/5.6, with quintuple single lens components as a convertible lens, for which greater sharpness of definition and intensity, with perfect freedom from astigmatism and distortion in the single lens, were claimed. It was issued in 1898, but, like an earlier analogous quintuplet of Messrs Turner & Reich (1895), it has not come into use on account of the cost and difficulty of construction. The latter firm, however, brought out in 1906 a new symmetrical quintuplet at f/6'8.

A triple anastigmatic combination containing remarkable new features, constructed and patented by H. D. Taylor, was issued in 1895 by Messrs Taylor, Taylor & Hobson under the name of the " Cooke Lens," and later by Messrs Voigtlander as the " Triple Anastigmat." It consists of three single lenses, two of them positive crossed lenses of crown glass with high refraction and low dispersion, with their most convex sides outwards, and between them, in front of the diaphragm, a single biconcave of light flint (fig. 39). All these lenses are designed to be free from diaphragm corrections, while the focal power of the negative lens is made as closely equal to the combined focal powers of the two positive lenses as may be FIG. 39. - " Cooke "Portrait Lens. FIG. 40. - " Cooke " Lens.

Aperture f/4.5. Series III.

necessary for the flattening of the field and correcting marginal astigmatism. They are not convertible, but arrangements are made for replacing the back lens by a low-power extension lens (Ph. Jour. 18 95, 1 9, p. 64). Series III., f/6.5 (fig. 40), and series IV., f/5.6, are portrait lenses. In the larger objectives of series II. the back lenses are adjustable for uniform sharp definition or a soft diffusion of focus. In a later series VI. (1907), f/5'6, this adjustment for diffusion is given to the front lens and is so arranged for portraiture that the diffusing adjustment and iris diaphragm can be operated from the back of the camera while viewing the focusing screen. A special fully corrected " Process " lens on the same general principle has recently been brought out for three-colour work and fine-line reproduction. Another distinctly new type of anastigmatic objective involving several new principles of construction was patented by H. L. Aldis in 1895, and brought out by Messrs Dallmeyer in three series, under the name of " Stigmatic " (Ph. Jour., 1896, 20, p. 117). It also approaches the triplet construction and depends on the introduction of air-spaces between the component lenses. According to Aldis, three conditions must be observed to obtain a flat field free from marginal astigmatism: (1) The converging lenses must be of high, the diverging of low, refractive index; (2) the converging and diverging components must be separated by a considerable interval; (3) thick meniscus glasses should be used. The first " Stigmatic " was a portrait lens, series I., 1896, f14. It has been made in two forms, first with a triple front lens, and a back negative system formed of a single thick crown lens of high refractivity with a negative ce mented meniscus. In the second form (fig. 41) the front component consists of a cemented positive and negative, and both parts of the back component are cemented lenses. All the converging lenses are of dense baryta crown, while both the diverging lenses in the back component are a light silicate crown. It is fully corrected for spherical and chromatic aberration, free from distortion and nearly so for astigmatism, giving equal illumination over a flat field of 60°. Diffusion of focus is obtained by unscrewing the back cell. Series II. (1897) is on the same principle but differs in construction, working at f/6 over an angle of 85° as a universal and convertible lens (fig. 42). The front or back component can be used alone, giving the choice of two focal lengths, 13 and twice the focal length of the complete lens. The principles of its construction were described by T. R. Dallmeyer in Ph. Jour. 1897, 21, p. 167. Series III., f17.5, will at f/16 give sharp definition over a plate two sizes larger. The single components are not convertible.

In 1897 Messrs Zeiss issued the " Planar," an objective of large aperture based on the principle of the Gauss telescope objective. It is a symmetrical doublet, each element consisting of three lenses, the two inner ones being a double convex and a double concave, of equal refractive but different dispersive power, cemented together and separated by an air-space from the outer convex meniscus (fig. 43). Its special points are its good colour correction, large relative aperture and intensity, varying from f/3.6 to f/6, with perfectly sharp definition and anastigmatic flatness of field over an angle of view from 62° to 72°. It is a very rapid wide-angle lens useful for instantaneous work with the cinematograph and hand cameras, also for portraits and groups, photo-micrography and enlargements or reductions (see E. Jb., 1898, p. 79, Von Rohr, p. 390, and Lummer, p. 81). Apochromatic planars with reduced secondary spectrum were brought out in 1903 for three-colour photography, and are also useful for astrophotography, the circle of diffusion being very small. The " Unar " (1900), f/4.5 in the smaller and f/6.5 in the larger sizes, angle 65° and 68°, was a further improvement by Dr Rudolph. It FIG. 41. - Stigmatic Portrait Lens. Series I.

?I. c? FIG. 42. - Stigmatic Lens. Series II.

consists of two unsymmetrical combinations, each formed of two single lenses of very transparent glass, dense baryta crown and light flint, separated by positive and negative air-spaces (fig. 44). The separate halves cannot be used as single lenses, neither being fully corrected for colour. It is well adapted for portraiture, groups or landscapes, especially for rapid hand camera work, on account of its covering power, with freedom from astigmatism and sharp definition with large relative aperture.

FIG. 43. - Planar. Series Ia. f/4. FIG. 44. - Zeiss's " Unar." In 1898 Messrs Goerz patented their " Double Anastigmat Celor," series Ib., f/4.5 to f/5.5. It is a symmetrical doublet, each element consisting of two thin single lenses: a positive of high and a negative of low refractive index, separated by an air-space (fig. 45). It is derived from the triple anastigmats by decreasing the refractive power of the central convex meniscus to the refractive power of air, so that it becomes a convex air-space between a double convex and a double concave lens. Less deeply curved surfaces can be given to the lenses, and the doublet gives anastigmatic flatness of field over an angle of 62° to 66°, equal to the best anastigmats with a still larger aperture. Series Ic., f/6.3, is similar and recommended for hand cameras, the aperture being smaller. Goerz's " Hypergon," (1900) 1/22, angle 135°, is a FIG. 45. - Goerz's " Celor." FIG. 46. - Goerz's " Alethar." symmetrical doublet of remarkable construction, consisting of only two single semi-globular, very thin lenses, with diaphragm at the centre of curvature between them. Astigmatism and curvature have been eliminated, and definition is good over the above wide angle with no distortion. Chromatic aberration is uncorrected, but compensated for by using a small stop. A star mask is fitted in front of the lens to allow for falling off of illumination towards the margin (E. Jb., 1901, p. 103). The " Syntor " (1903), Series Id., f/6.8, angle 64° to 70°, is on the same principle as the " Celor," but cheaper, for use in hand cameras or telephoto combinations. The " Alethar," series V. (1903), fir', is a lens with diminished secondary spectrum, for three-colour reproductions, half-tone process work, and general purposes. It is a symmetrical doublet, each element consisting of a negative and positive separated by an air-space (fig. 46). The negative is composed of three cemented lenses, which correct the spherical and chromatic aberrations more fully than hitherto possible, so that all the colours of the spectrum are focused in the same invariable plane. It gives great crispness of definition at full aperture (W. Zschokke, E. Jb., 1904, p. 165). Goerz's " Pannar," f/6.3 (1904), is a convertible 4-lens anastigmat, and an improvement on the " Dagor," in that the single elements are completely corrected for coma, and thus form efficient long-focus lenses for landscape, &c., at an aperture of f/12.5, while the doublets formed by various combinations of the single elements are universal objectives working from f/6.3 to f17.7. The single elements are similar to those of the " Dagor," but have an additional negative lens at the back, so that the outer two of the three cemented surfaces have a collective and the inner one a dispersive action, by which coma is eliminated (E. Jb., 1905, p. 55).

In 1902 H. L. Aldis issued the " Aldis Lens," f/6, a doublet composed of a cemented meniscus in front and a single double convex back lens. It is a long-focus objective with short back focus, and is made in two forms, series II., f/6 (fig. 47), and series III. (1903), f/7.7 (fig. 48). In the latter the back element is very thin, and the front combination of infinite focal length. By discarding the symmetrical form simplicity is secured, while open or reflecting surfaces are avoided. Special attention has been paid to perfect correction of spherical aberration in the centre of the field. It is lighter, smaller and cheaper than series II. The " Duo " lens of the same maker (1907) is intended to replace the front lens and double the focus, but with less rapidity and without any loss of quality. The " Trio " (1908) is similar, but only increases the focus one and a half times and is thus more suitable for cameras of short extension. The Aldis " Oxys " anastigmat, Series II. (1908), f/5.65, angle 85°, is an improved form. Being an unsymmetrical cemented doublet it is free from the defects incidental to air-spaces and is constructed to give more perfect correction for flatness of field with large aperture and wide angle.

It is generally stated that it is impossible to make a spherically, chromatically and anastigmatically corrected photographic objective with the old optical glasses. K. Martin, of Messrs Busch of Rathenow, has, however, shown (E. Jb., 1902, p. 68) that it is quite possible to do so with a system of separated lenses, and that it is immaterial whether the index of the flint or the crown is the higher. An anastigmat on this principle was issued by Messrs Busch in 1902, as the " Omnar," series III., f/7.7 (fig. 49). Series II., f/5.5, angle 75°, and I., f/4.5, have since been issued. It is a symmetrical FIG. 49. - " Omnar," FIG. 50. - Ross's Series III. " Homocentric." doublet, each element consisting of a negative flint meniscus of higher refraction, and a positive crown of lower refraction with an air-space between them in the form of a negative lens. The back element can be used alone. The " Lumar " series, by G. Rodenstock, is similar. In 1902 Messrs Ross brought out the " Homocentric," a symmetrical doublet, each element consisting of a negative and positive meniscus separated by an air-space (fig. 50). It is constructed so that all rays of light emanating from an y one point of the object are converged again into one point in the image. It is also quite free from spherical zones, is not altered in focus with different diaphragms, and thus has exquisite defining power. The colour correction is so perfect that the different coloured images are identical in size and position, thus rendering it specially suitable for three-colour and process work. The back lens can be used alone, with diaphragms, as a single lens of about double the focus of the doublet. It is made in several series: II., f/5.6, and III., f/6.3, for rapid and instantaneous work; V., f/8, for ordinary purposes; VI., f/8, for process work and three-colour reproduction. A later series, IV. (1907), " Compound Homocentric," f/6.8, differs from the others in being a symmetrical doublet composed of two triple cemented elements, very close together and separated by a diaphragm. It is FIG. 51. - Zeiss's FIG. 52. - Voigtlander's " Tessar." " Heliar." specially suitable for outdoor work, also for copying and enlarging, having good covering power. Zeiss's "Tessar " (1902) is a rapid unsymmetrical doublet, formed of two separated uncemented positive and negative lenses in the front element and a cemented meniscus at the back (fig. 51). The two halves cannot be used separately. The glasses used are very transparent, permanent and lessen the secondary spectrum. Three series are made by Messrs Ross, Ic., f/3.5 for cinematographic work and portraiture, and f/4.5 for handcamera work and portraiture; IIb., f/6.3 for general purposes, and VIII., the " Apochromatic Tessar," specially corrected for threecolour work and reproduction. They all give fine definition over a large flat field, free from any zonal aberration. The f/3.5 portrait lenses, with double the field and covering power of the Petzval lens, are anastigmatic and free from distortion. Messrs Voigtlander's " Heliar" (1902), f/4.5, angle 50°, calculated by Dr H. Harting, is an objective of large aperture, suitable for portraits and very rapid instantaneous work, being well corrected for astigmatism, coma and curvature of field, with freedom from flare. It is a triplet consisting FIG. 47. - Aldis Lens. Series II.

FIG. 48. - Aldis Lens. Series III.

of a central negative lens, with cemented double front and back lenses (fig. 52). The negative lenses are of light silicate flint, the two positive of the heaviest baryta crown. Besides being a rapid universal lens, it is specially suitable for half-tone process work, with a large diaphragm (E. Jb., 1903, p. 117). The " Dynar " (1903), f/6, angle 60°, is of somewhat similar construction, but differs from the " Heliar " in the positive lenses of the cemented pairs being outside instead of towards the central lens. It can only be used as a whole.. It is made of hard colourless Jena glasses, giving great brilliancy and uniformity of illumination over a large angle, and is specially adapted for very rapid hand-camera work.

Dr R. Steinheil's " Unofocal " (1903), f/4.5 is a symmetrical doublet, each element consisting of two single separated lenses of equal refractive power and of equal focus of opposite signs, hence its name. Each half can be used as a single lens with small stops. In its construction a quite new principle was followed, the separation of the lenses fulfilling an important part in the colour correction, as explained by Conrad Beck in Ph. Journ. (1904), 44, p. 1 77. This plan satisfies the Petzval condition and removes its restrictions, so that a lens of f/4.5 can be produced with telescopic central definition, perfect freedom from distortion and flare over a flat field of 60°, with great equality of illumination (fig. 53). They are made by Messrs Beck in two series: II., f/4.5, for portraiture, rapid handcamera work, telephotography and projection; and I., f/6, in which the lenses are closer together, for hand-camera work and general purposes. E. Arbeit's "Euryplan " anastigmats (1903), made by Schulze Bros., Potsdam, are apochromatic objectives of quite new construction, giving perfect definition with large apertures over a FIG. 53. - Beck-Steinheil FIG. 54. - Euryplan, " Unofocal." f/4'5.

wide angle, made in four series: I., f/4.5, angle 80°; II., f/5.6, angle 90°; III., f/6.8 to 7.5, angle 82°; IV., f/6.5. They are symmetrical doublets, each element consisting of three lenses, a new achromat formed of a biconvex of heavy baryta crown of high refractivity and low dispersion, separated by an air-space from a positive meniscus of the same baryta crown, with its concave side towards the diaphragm. In series I., f/4.5, the two positives are placed outside (fig. 54), in series II. and III. they are inside. The single elements are fully corrected astigmatically and chromatically, and can be used singly at double the focus (E. Jb. 1904, p. 35).

Beck's " Isostigmar " (1907) is a new anastigmat showing a distinct departure from the ordinary principles of construction, in that it does not fulfil the Petzval condition that the sum of the focal powers of its individual lenses multiplied by the reciprocals of their respective refractive indices should be equal to zero, or E (I / A O =0. It is a 5-lens combination, two separated thin single lenses in the front element and three in the back (fig. 55). In departing from the Petzval condition very low power lenses can be used, thus reducing the initial errors to be corrected; no individual component having a shorter focal length than one-half that of a complete objective. A special feature is the excellent correction of the oblique spherical aberrations and central aberrations, giving a practically flat field without astigmatism over angles from 60° to 90°. The half combinations can also be used alone with diaphragms a long focus lenses of different foci (Ph. Journ. 1907, 47, p. 191). It is issued in six series: I. (1908), f/4.5, large aperture, series, for reflex press work and portraiture; Ia., f/6.5, angle 60°-65°, latter is very useful when an extended use of the rising front is required, either at a wide or ordinary angle. V. (1908), f /11, " Process " lenses specially corrected to give a flat field for copying. They can be fitted with suitable reversing prisms. VI. (1908), f/5.6, variable portrait lenses, adjustable for sharp or soft definition from the back of the camera while focusing.

The above represent the principal types of anastigmats, but many more objectives of the kind, triple or quadruple, cemented or uncemented, with air-spaces, in many modifications, have been issued by English and foreign makers.

6. Telephotographic Objectives. - For some years past special objectives, or attachments, have been constructed for photographing near or distant objects on an enlarged scale with an ordinary camera, the extension required being very much less than would be needed to obtain an image of the same size with an ordinary long-focus lens without enlargement. They consist of a combination of a positive converging with a negative dispersing lens, by which the image is picked up and enlarged to varying degrees, according to the system of lenses used and the extension given to the camera, thus producing the same effect as a positive lens of very much longer focus. Enlarged images of this kind can also be made by a combination of two converging lenses, one of them forming an image of the object, which is received on the other of shorter focus and projected on the sensitive plate, being enlarged more or less according to the optical conditions and relative positions of the lenses and sensitive plate. The photoheliographs at Greenwich and other solar observatories, designed by Warren de la Rue, are on this principle. Portable apparatus of the kind was made in 1869 by MM. Borie and de Tournemire, and later by Jarret, but this system requires much greater extension of the camera, entailing more loss of intensity of the image, and has never come into use.

The modern telephotographic combination is generally looked upon as an application of the principle of the " Barlow " lens, but it really goes back to the Galilean telescope (c. 1610). J. B. Porta mentions the combination of concave and convex lenses for giving enlarged and clearer images of near and distant objects (Magia Naturalis, lib. 17, cap. 10, 1589). J. Kepler showed that by a combination of a convex with a concave lens images of objects could be depicted on paper of a larger size than by the convex lens alone, but reversed (Dioptrice, Prob. cv. 1611). Christopher Schemer made use of the same principle in his " Helioscope " for solar observations (Rosa Ursina, cap. vii, 1630). F. M. Deschales and P. Z. Traber also dealt with the question, and in J. Zahn's Oculus artificialis Teledioptricus (1686) we find figured a reflecting camera fitted with a compound enlarging lens on this principle. In his Nova Dioptrica (1692), W. Molyneux has given some interesting problems for calculating the position of the compound focus of a convex with a concave lens, also the angles subtended by an object on the focal plane. If for the simple uncorrected glasses then used we substitute a system of photographically corrected positive and negative lenses, suitably mounted, and put a sensitive plate in place of the paper, we have the modern telephotographic arrangement. I. Porro seems to have been the first to use a combination of this kind for photographing an eclipse in 1857, and later for terrestrial objects. It consisted of a small achromatic single lens combined with a concave lens. Many attempts were afterwards made in France, and also in England, to utilize the principle, but special lenses for the purpose were not available. Ad. Steinheil constructed one in 1889 for the Brussels Observatory, and another in 1890 for the Marine Department in Berlin. In 1891, curiously enough. three such combinations were worked out quite independently and patented, by T. R. Dallmeyer in London, A. Miethe in Berlin and A. Duboscq in Paris. Since that time these combinations have been greatly improved by increase in the working apertures and reduction in size and weight, so that they can be used in hand cameras. They are exceedingly valuable for obtaining details of inaccessible objects at a distance, whether architectural or topographical, and for photographing animals without approaching them too closely. Large portraits can also be taken with much better perspective effects and more conveniently than by using long-focus lenses much nearer to the sitter. With the very perfect telephoto FIG. 56.

long focus, for portraiture, &c.;: II., f/5.8, angle :70°, for general use, III. f/7.7, angle 65°, similar to II. but less rapid; IV. f/6.3, angle 90°; wide angle, giving satisfactory definition at full aperture over an angle from 80° to 85°. Having such a large reserve of covering power the graphic objectives now available the loss of intensity of illumination, which no doubt was the bar to early progress in this direction, has been overcome, and definition has also been improved, so that snapshots can readily be made with combinations of high intensity, while with those of ordinary intensity the exposures are not unduly prolonged, and good definition can be obtained over an extended field.

The optical principle on which these combinations are based is very simple, and will be understood from fig. 56. It depends mainly on the fact that in order that a real image may be thrown on the screen of an object AB, the rays proceeding from it, which pass through the positive system L 1, must come to a focus at a point f within the secondary focus f" of the negative system L2. Falling within this limit, they will be intercepted by L2 and made less convergent, so that instead of coming to a focus at f, they will continue to converge till they reach the screen at f", and will there form a proportionally larger image a'b of AB than the image ab given by the positive lens alone at f; just as stated in Kepler's problem. Moreover, this image a'b will be of the same size as if it had been produced directly by a positive lens L3 with a focal length equal to if"', and this distance is the equivalent focal length of the entire system. It can be found from the formula F = f, f2/d, where _A and f2, are the focal lengths of L 1 and L2 respectively, and d=fv-ff 2 - s, s being the distance between the lenses. In many instruments of the kind a scale showing the value of d is engraved on the mount. If the rays from AB come to a focus in front of L2, on it, or beyond f", no real image can be projected on the screen. There is therefore a certain limit, which is greater in proportion to the length of focus of the negative system, within which the focus of the positive system L 1 may fall and produce a series of well-defined images on the screen, which can be varied in size by altering the amount of separation of the two systems of lenses within the above limit, and the distance of the screen from L2. Every change in the position of the screen will involve a corresponding adjustment of the lenses. The greater the extension of the camera and the closer the lenses, the greater the size of the image, and vice versa. The camera extension for a given magnification can be found by multiplying the focal length of the negative system by the number of magnifications, less one. The magnification produced by a given camera extension is found by dividing the latter by the focal length of the negative system, and adding one.

In its usual form (fig. 57) the telephotographic combination consists of a quick-acting portrait lens, or an anastigmatic doublet of FIG. 57. - T. R. Dallmeyer's Compound Telephotographic Lens.

large aperture and relative intensity of suitable focal length, fitted at one end of a tube, in which slides a smaller tube carrying a properly corrected negative system, which may vary in focus, but must be of shorter focus than the positive (usually about half); the shorter the focus the greater the magnifying power for a given extension of camera. The amount of separation of the lenses is limited on the one hand by the position of the focus of the positive system, and on the other by the focus of the negative system, as explained above, and can be adjusted within these limits by a rack and pinion. The tubes are adjusted so that when closed up the two foci may coincide, or nearly so, and d = o, or its minimum value; and when opened to their fullest extent the focus of the positive may fall upon the negative system, or so that d may not exceed the focal length of the negative system. Within these limits the focal length of the combination will be positive, and a real image formed on the screen. Several forms of them have been brought out by various makes, some, as Zeiss's, with a special positive lens, others for use with anastigmats and other lenses of large apertures. The negative lenses are also made of various powers.

Messrs Dallmeyer's " Adon " (1902) is a telephotographic lens, for use with hand cameras, composed of two achromatic combinations adjusted for parallel rays, a front positive lens 4? in. focal length, and a back negative lens of 24 in. focus. These are mounted to permit of great variation in the separation, so that when the " Adon " is fixed on the front of a suitable lens, near or distant objects may be taken on an enlarged scale without altering the focus of the camera, or the enlargement can be varied with further extension of the camera. Used alone it is a complete telephoto lens of moderate magnifying power, and will cover plates 15 in. X 12 in. In 1903 a special form, the " Junior Adon," was made in three kinds for use with kodaks and similar folding hand cameras, single and double extension, giving a fixed degree of magnification without loss of rapidity, while focusing can be effected by scale. It is intended to replace the front lens of an R.R. or anastigmatic lens and cannot be used independently. Messrs Busch's " BisTelar " f/9 (1905), is another compact fixed focus telephoto lens, specially for use with hand cameras. It is a complete lens in itself, requiring no attachments and can be fitted to a central shutter. It is made in three sizes magnifying from two to three times. An improved form of this lens (1908), working at the large aperture of f/7, is similar to an old form of " Dialytic " lens worked out by J. Petzval, having a positive front and negative back meniscus, with their concave surfaces facing inwards (fig. 58). As in the old " Orthoscopic " and lenses of that type there is some outward distortion, but it is very slight. These lenses are made in five sizes with foci from 8 to 22 in., requiring camera extensions from 41 in. to t IIin. They magnify about twice.

According to K. Martin, a telephoto combination of the Bis-Telar type can be used in a reversed position as a projecting lens for the lantern, with the advantage of increasing the illumination from a given source of light (E. Jb. 1908, p. 46).

Captain Owen Wheeler proposed in 1907 a high-power telephoto arrangement, made by Messrs Staley, in which the negative attachment consists of three negative lenses, any single one of which can be used separately, giving magnifications of about 6, 9, and 13 diameters with a camera extension of 14 in. By combining the three a magnification of 30 diameters is attainable with the same short extension, which is a great advantage in many ways. In 1908 Messrs Zeiss issued their " Special Tele-objectives " in two sizes working at f/Io, the larger with an aperture of 3.14 in. and 32 in. focal length fitted in a special " tele-camera " for plates 9 X 12 cm. with a monocular field glass magnifying four times as finder. The smaller one, with 18 in. focus, is adapted for hand cameras with 6 in. bellows extension. They consist of specially corrected positive and negative combination with a definite focal length and requiring a definite camera extension, and are specially suitable for balloon photography, instantaneous portraiture, &c. The theory, construction and use of telephoto lenses has been fully described by T. R. Dallmeyer in his Telephotography. 7. Anachromatic Lenses. - For large portraiture a certain amount of softness and diffusion of the image has long been recognized by artists as desirable, and in 1895 the " Dallmeyer, Bergheim Lens " was constructed with this special object. It is composed of a single uncorrected positive meniscus front lens, with a diaphragm in front of it, and an uncorrected negative meniscus back lens, and in the larger sizes it has great range of focal length on the telephotographic principle. The spherical and chromatic aberration produced by the uncorrected single lenses gives the diffusion of focus which produces the peculiarly soft and delicate effect aimed at. It is most useful for large heads and life-size studies, the great depth of focus conducing to uniformity of definition. There is no distortion, and by stopping down to about one-third perfect definition can be obtained. It works with great brilliancy, both elements being single glasses. It was the first of the anachromatic portrait lenses. Since 1903 Messrs C. Puyo and L. de Pulligny have been experimenting with various combinations of uncorrected lenses for producing the same effect in portrait and landscape photography by the diffusion of focus produced by chromatic aberration, and suitable lenses of this kind have recently been brought out in Paris as Les Objectifs d'artiste In their construction the principal points to be considered arc spherical aberration, to be minimized in the form and arrangement of the lenses selected; distortion, corrected by using a symmetrical system; astigmatism, avoided by using combinations of low power. The lenses used by Puyo have been: (I) a piano-convex crown with convex side in front at f/8 or f/9, or even f/5 for heads; (2) a simple thin concavo-convex meniscus, with concave side in front, is better and suitable for full lengths at f /Io; (3) a symmetrical system formed of two similar crown menisci, concave sides inwards, is generally useful when worked at f/to, or even f15. Arrangements are made in mounting these lenses for automatically making the necessary correction for colour. Another form is the " Adjustable Landscape Lens," formed of an anterior plano-convex crown, 3 cm. diameter, and a posterior plano-concave crown, each of 10 cm. focus, and the same radii of curvature. In contact the y have an infinite focus, but when slightly separated any focus can be obtained up to about 10 cm. In such a telephotographic system, properly stopped down, anastigmatism, flatness of field, and rectilinearity are secured over a fairly large field. These lenses are fully described in Les Objectifs d'artiste, by L. de Pulligny and C. Puyo (Paris, 1906), and various forms, portrait and landscape, have been made by Messrs Hermagis, Turillon & Morin (see Fabre, T. E. P. Suppl. D. IoI).

In order to regulate the intensity of the illumination by the lens, to enlarge its field, and, in the case of the older forms of objectives, to extend the area of good marginal definition, diaphragms are used, usually with circular apertures. They are made in different ways: (I) as single metal plates, fitting into a slot in the lens tube (Waterhouse diaphragms); (2) Rotatory: a single plate revolving on a central axis and pierced with apertures cut to fit centrically in the opening of the lens; (3) Iris: a form of diaphragm now very generally used, and very convenient, because it can be easily adjusted as required for intermediate apertures. As a rule they are placed at the optical centre between the elements of a compound lens or in front of a single one.

In order to provide a uniform system of diaphragm apertures, X.XI. 17 FIG. 58. - " Bis-Telar." the Royal Photographic Society in 1881 drew up some rules, which were revised in 1891 and again in 1901. The former standard unit f /4, and the numerical notation used with it, have been abolished in favour of the unit f/1 established at the International Congress in Paris 1900. Intensity ratio is defined as dependent upon the effective aperture of a lens, and not upon the diameter of the diaphragm in relation to the focal length of a lens. The effective aperture of the lens is determined as follows: The lens must be focused for parallel rays. An opaque screen is then placed in the principal focal plane, and a pinhole is made in the centre of the plate (in the axis of the lens); an illuminant is placed immediately behind the pinhole itself, when the diameter of the beam emerging from the front surface of the lens may be measured. (It will be found that except in the case of the diaphragm being placed in front of the lens, the diameter of the diaphragm itself is seldom that of the effective aperture.) Every diaphragm is to be marked with its true intensity ratio as above defined, but the present intensity ratios are retained in their order of sequence: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f18, f/11.3, f/16, f/22.6, f/32,f/45.2, f/64, &c., each diaphragm requiring double the exposure required by the preceding one. In other cases apertures are to be made in uniformity with the scale, with the exception of the highest intensity, e.g. a lens of f/6.3 would be marked for f/6.3, f/8, &c. The corresponding numbers are known as f numbers, but are only applicable for a lens focused for distance. Other systems of notation are in use, but the above is generally adopted (see Fabre, T.E.P. Sup pl. C. 38). Special diaphragms are in use for process work with ruled screens (see N.S. Amstutz, Handbook of Photoengraving, 1907). Standards for the screws of photographic lensflange fittings, and for the screws fitted to cameras for attachment to the stand or for fixing movable parts, have also been laid down (Ph. Journ. 1901, 25, p. 322).

Instantaneous Shutters. The general use of rapid dry plates and hand cameras has rendered it necessary to have some mechanical means of regulating exposures in small fractions of a second, especially for objects in rapid motion, and this instantaneous shutter has become an essential part of modern photographic equipment in many forms and patterns, but practically three types are preferred - the between-lens shutter, the roller-blind shutters, used before or behind the lens, and the focal plane shutter, in front of and close to the plate and forming part of the back of the camera. The usual limit of rapidity of the two former is nominally about ifoof a second, and for ordinary pure poses higher speeds are seldom required, while with the latter speeds of itoa to zAo of a second may be attained.

Two important factors in the use of lens shutters are the rapidity or speed, measured by the total duration of exposure from opening to closing, and the efficiency, measured by the ratio of the time during which the shutter is fully open and the time occupied in opening and closing. Both factors are more or less variable, either with differences of construction, of diaphragm opening or of position of the shutter with regard to the plate and lens. In any case the efficient exposure is always less than the actual, and may be considerably so.

The rapidity required of a shutter in photographing moving objects is regulated by the minimum time necessary to produce a well-exposed image upon the plate, with a loss of definition, or blurring, by displacement not exceeding i oo, or preferably to z o o of an inch, if enlargement is extended. This will depend on the state of the light and the illumination of the object, the relative intensity of the lens as measured by its effective aperture and focal length, the sensitiveness of the plate, and the amount of effective light passing through the shutter during the exposure. The amount of displacement to be guarded against depends upon the rate of movement of the object, the direction in which it is moving with reference to the axis of the lens, its distance from the camera, and the focal length of the lens. It will be proportionately less as the distance of the object increases, and as the rate of its motion and the focal length of the lens for a given distance decrease, and vice versa. It will be greatest when the object is moving at right angles to the axis of the lens, and least when the motion is directly towards it; but in that case there will be some increase in the apparent size of the object as it approaches the camera. For example: An object moving 1 m. an hour advances 17 . 6 in. per second. With a lens of 5-in. focus this would represent a displacement on the ground glass, for an object 50 ft. away, amounting to 146 in. per second, and it would require exposures between T i g and, of a second to give maximum or minimum displacements of the image between i 01 and zko of an inch. An object at the same distance moving ten times as fast would require 1-10 of the above exposures. If, however, the distance be increased, the possible exposure may also be increased in the same proportion, so that the object moving 10 m. an hour at 500 ft. distance would only require the original exposures of i z g to 3 1 - 7 of a second. On the other hand, the limits of exposure for an object moving 1 m. an hour within t o ft. of the lens would be between 7N and 1 of a second. This is entirely independent of the sensitiveness of the plate, and only represents the maximum duration of exposure permissible in order to reduce the blurring of the image between certain limits. The sensitiveness of the plate, and the intensity and amount of light acting upon it through the lens and shutter, must be adjusted so as to produce the desired photographic effect within that time. With a lens of 8 in. focal length the displacement would have increased in the first instance to 23 in. per second, and the maximum exposure permissible would be from 2 to 7 of a second. This shows that there is an advantage in using short-focus lenses for very rapid exposures. In practice, most work of this kind is done upon quarter-plates (44X34 in.) with lenses of 41 to 5 in. focus. As the displacement will be greatest for an object moving at a right angle across the axis of the lens, an exposure sufficient for this case will be sufficient for any other. Sir William Abney has discussed this question practically in his Instantaneous Photography, and it is treated mathematically by W. B. Coventry in his Technics of the Hand Camera, in which will be found formulae and tables for ascertaining the distances and limiting exposures for moving objects, allowing for a blur of -d--00f an inch. In foreign treatises the limit is usually calculated for a displacement of o of a millimetre, or about t, of an inch.

An efficient shutter should fulfil the following conditions: It should be light and compact, simple in construction and action, strongly made, and not liable to get out of order; capable of being set without admitting light into the camera; easily released with a slight pressure of the finger, if a pneumatic release is not fitted, and free from any tendency to shake the camera on release. It should open and close quickly, allowing the largest possible proportion of the exposure to be made with the full aperture, and it must not cut off any of the effective light passing through the lens, but should distribute it evenly all over the plate: though in landscape work it is an advantage to give the foreground more exposure than the sky. It should be adjustable for variable instantaneous and for prolonged or " time " exposures. With a good shutter there is less risk of shaking the camera in short " time " exposures, from 4 to 1 second, than there is in taking off a cap. Shutters working between the lenses must permit of the use of diaphragms in the lenses, and of alterations of speed while set. Above all, a shutter must be constant in its action, giving short and variable exposures always correctly or relatively so, an important condition which cannot always be fulfilled, and the exposures marked on the indicator should be capable of being repeated with tolerable certainty. Shutters should also be adaptable for use with different lenses. Three methods of varying the speed of a shutter are in use: (1) by altering the length of the slot; (2) by the retarding action of a pneumatic brake; (3) by varying the tension of a spring. The latter is considered by W. B. Coventry as far the best. They are usually released by the pressure of the finger on the end of a lever holding the moving part in a state of tension; or better, by J. Cadett's system of pneumatic pressure, applied by means of a compressible rubber bulb and tube, which may drive a piston acting on the lever holding the shutter, or inflate a collapsible bulb at the other end of the tube and thus exert the necessary pressure:on the lever. With W. Watson's " Antinous " release a flexible wire acts directly on the piston or trigger release of a cylinder shutter. It is also adapted for roller-blind, focal plane, flap, and various forms of between-lens shutters. It is durable, effective and convenient (see fig. 3). In many cases both methods can be used as desired, the mechanical release being preferable on account of its convenience and freedom from liability to shake the camera.

The following are the principal types of instantaneous shutter: (1) Flap, (2) drop, (3) combined drop and flap, (4) rotary, (5) roller blind, (6) focal plane, (7) moving blade central, (8) iris. They can be applied in four different positions: (a) in front of the lens; (b) centrally, near the diaphragm; (c) behind the lens (d) immediately in front of the sensitive plate. They all, however, come under two main classes: Lateral, including those in which the exposure commences and ends at the circumference of the lens aperture; and Central, those in which the exposure begins and ends at the centre of the aperture. Some of them are " lateral " in their single form and " central " when double. The form and position of the effective aperture of a shutter, relatively to the lens and plate, have a strong influence, either favourable or unfavourable, on the amount of effective light passing through the lens, and its even distribution over the plate. This is especially the case during the incomplete phases of opening and closing the aperture. It seems to be agreed that the best position for lens shutters of the lateral type is behind the objective, and for those' of the central type, between the component lenses. In this latter position the whole of the plate is illuminated during the full period of exposure, with a gradually increasing intensity, until the full opening is reached, and then the illumination gradually falls off until the shutter is closed. The most effective shutter is one in which the first and third phases of incomplete illumination, during the opening and closing, are the shortest compared with the second phase of full opening.

With the focal plane shutters, however, different portions of the plate are exposed in succession, the lens working at its full aperture and efficiency throughout the exposure.

To secure successful results in using instantaneous shutters, the operator should make himself acquainted with the working of his shutter and its efficiency in various circumstances of exposure with the lenses, plates and developer he proposes to use ascertaining the actual value of the various exposures marked on the indicator, and, what is more important, how far they can be depended on for regularity. There are many simple ways in which the actual time of exposure from opening to closing can be ascertained sufficiently closely for practical purposes. They depend upon the measurement of the trace left on a sensitive plate by the passage of a brightly illuminated object revolving at a known speed or falling vertically through a known distance, when photographed with different speeds of the shutter against a dark background. These, and the more elaborate methods for obtaining more accurate determinations of the shutter-exposure periods and of the corresponding effective exposures - i.e. showing the actual effect of the shutter through its different phases from opening to closing - have been described by Sir William Abney in the work already mentioned, by A. Londe in La Photographie moderne and La Photographie instantanee. An apparatus for testing shutters at the National Physical Laboratory was described by J. de Graaf Hunter in the Optician, 1906.

1. Flap Shutters. - The simple flap shutters consisting of a hinged flap opening upwards in front of the lens, though favourites in early days for landscape work, and still useful for intermittent exposures or as sky-shades for securing cloud effects or increasing foreground exposures, have been almost superseded by quicker and more compact forms. They are used with single and double flaps for portraiture and studio work, for which purpose they are made to act noiselessly and not attract the attention of the sitters. Guerry's (figs. 59 and 60) is a good example of the type. W. Watson's " Silent " FIG. 59. - Guerry's FIG. 60. - Guerry's Single-flap Shutter. Double-flap Shutter.

shutter is hemispherical in form and collapsible, the two wings opening out and folding together, when actuated by a special " Antinous " release, and R. & J. Beck's is another form, a single lifting flap with pneumatic release.

The old simple drop shutter, in which a plate having an opening in it falls in front of the lens aperture, has been superseded by the more compact and quicker-working roller-blind shutters, which act on much the same principle. It had a theoretical interest in connexion with the effect of different forms of aperture - circular, square, or elongated - used with shutters of the lateral type, but it is now generally recognized that a more or less extended rectangular opening, of at least the full width of the tens aperture, is best for securing the even admission of light from all parts of the image with shutters of the rectilinear lateral type, to which this and similar shutters, in which a single opening passes across the lens aperture, belong. In Busch's " sky shade " shutter (1907), fitting on the front of the lens a single leaf moves vertically upwards and descends again, giving less exposure to the sky.

In early dry-plate days several forms of this kind of shutter were brought out, under the names of Phoenix, Phantom, &c., but are now little used. In these shutters, in addition to the drop slide, there was also a lifting flap, which on release opened from below, and, having fully uncovered the aperture, released the drop slide, which fell and closed the shutter. They were useful and effective in the smaller sizes, but heavy and cumbrous in the larger. Speed could only be estimated very roughly by the use of india-rubber bands for giving tension.

These are of the lateral type, and consist of a circular metal disk revolving on an axis eccentric to the axis of the lens, and furnished with a radial sector-shaped opening, which passes laterally in front of the lens aperture when the tension of a spring is released (fig. 61). They are used in various patterns in cheap hand cameras, usually in front of the objective, though they may be placed behind it or between the component lenses. So long as the opening is at least equal to the size of the lens aperture, the illumination is sufficiently even, but the openings are usually elongated so as to give a longer period of full opening. Working by a spring they are more portable and convenient than drop shutters. Beck's FIG.61. - RotaryShutter. " Celverex " between-lens shutter (1906) is of this type, the disk being revolved by a spring and the variations of exposure obtained by altering the size of the opening passing over the lens aperture, and not the tension of the spring. It is speeded for exposures of 4 'o sec.; also " bulb " and " time." It is fairly accurate and consistent in action, but loses efficiency at the highest speeds by the diminution of the opening.

5. Roller-Blind Shutters. - For general use the well-known rollerblind shutter of the single lateral type, as made by Thornton-Pickard and others, is undoubtedly one of the most popular and efficient. It possesses most of the qualities laid down as essential to a good shutter, gives good illumination, appears to be fairly regular in its action, and can be used for time or instantaneous exposures. It consists of a light mahogany or aluminium box, arranged so that it can be fitted in front of or behind the objective. It is made in different sizes, and each size can be adjusted to smaller objectives (fig. 62). It is also made with a disappearing cord, and in an improved pattern, the " Royal," all the fittings are inside the box. By pulling the cord FIG. 63. - Mechanism of the Thornton. Pickard Roller, Blind Shutter.

A, Upper roller.

B, Lower roller.

c, Cord.

D, Black curtain.

, Aperture in curtain. R, Rubber ring adapter.

an opaque black curtain with an elongated rectangular aperture is unrolled from the lower roller on to the upper one, and held by a coiled spring on the lower roller (fig. 63). Pressure on a pneumatic bulb inflates a second smaller bulb, raising a lever which releases the spring, and thus brings the blind down with a rapidity which can be adjusted by turning a handle actuating the spring, the corresponding speed being shown on an indicator. For time exposures, pressure on the bulb opens the shutter, and another pressure closes it, but an arrangement is now made by which time exposures of a, 4, z, I, 2, 3 seconds can be given automatically, the pressure of the bulb opening the shutter, which closes of itself at the expiration of the exposure required. The theory of shutters of this type has been very fully discussed by Coventry (op. cit. p. 50), who shows that for any given tension of the spring the actual exposure decreases as the size of the lens aperture diminishes, while the effective exposure remains constant for all apertures. This is peculiar to the lateral shutter. He also shows that with plates of very different rapidities, though the exposure may be the same, the actual exposure effective is less with the rapid plate and a small stop than with the slow plate and a large stop; consequently the blur due to the movement of the object would be proportionately less on the rapid plate than on the slow one. Also that for any given lens the smaller the shutter the more rapid toe exposure can be made, though with the same lens a larger shutter is capable of giving a more efficient though less rapid exposure. It is better, therefore, for moderate exposures, to have a larger shutter than the size of the lens requires. Sir William Abney had given diagrams of the action of a shutter of this kind in his book referred to; they show clearly that the centre of the plate gets more exposure than the margins; but practically this is not very noticeable, and the action is very regular.

6. Focal Plane Shutters. - These are also roller-blind shutters with mechanism similar to the foregoing, but arranged so that the slit in the curtain may move rapidly close in front of the sensitive plate, exposing different portions of it in turn, the intensity of the exposure being regulated by the width of the slit, whether adjustable or not, and the rapidity with which it is moved by the unwinding of a spring. The advantages of these shutters are now being fully appreciated, the principal being that they are quite independent of the lens, so that one shutter will serve for different lenses, and any suitable lens may be used at its full intensity, without the loss of efficiency inherent in the ordinary forms of lens-shutters. They thus add effectively, if not actually, to the speed of a slow lens, or if a lens be stopped down there is less loss of efficiency, with a gain in increased depth and definition. They are particularly well adapted for the very short exposures required in photographing near and quickly moving objects, racing horses, divers, &c., and many reflex and other hand cameras are fitted with them. They are constructed in different forms, either for short exposures with high speeds alone, or for short and prolonged exposures; with a single slit of fixed or variable width moved at regulated speeds, or with a series of slits or openings varying in width, their speeds being adjusted by the FIG. 62. - Thornton-Pickard Roller-Blind Shutter with automatic exposure appliance.

tensions of the springs. Thus the new Goerz-Anschiitz shutter has ten tensions and nine curtain apertures, providing for ninety differ ent. speeds or exposures, rang- " ing from i o to TI 'sa of a second, besides autobulb exposures from 2 to 5 seconds and time exposures (fig. 64). Most of these shutters are now provided with a self-capping device for protecting the sensitive plate during the setting of the shutter. As the slit moves progressively over the plate, if it is too narrow or moving too slowly, it may cause distortion of the images of quickly-moving objects, especially if near the camera, but with due care in regulating the width of the slit and the duration of exposure this is practically not often percepFIG. 64. - Goerz-Anschiitz Focal table, especially if the slit is Plane Shutter. arranged to move in the same direction as the object.

The theory of these shutters is discussed by Coventry (op. cit. p. 69), more fully by Fabre (T. E. P. Suppl., C. p. 128), and their practical use in Focal Plane Photography (" Photo-Miniature Series," No. 77, 1907).

7. Moving Blade Central Shutters. - These shutters, in which two thin metal or ebonite plates or opaque curtains with round or rectangular apertures, or in other cases two curved blades, pass very quickly over each other in opposite directions, are largely used in many patterns fitted between the lenses of a combination or attached to them in front or behind. Formed of two single lateral shutters opening and closing in the centre of the lens aperture, they become central, the exposure taking place during the short period in which the openings are passing each other or the curved blades opening out and closing again. To obtain the greatest efficiency the size of the openings should correspond with the full aperture of the lens. If each plate moves as fast as a drop shutter the combination gives double the speed, corresponding to half the exposure. The sensitive plate will be most evenly and strongly illuminated when the leaves of the shutter work inside the lens near the diaphragm, as in Bausch and Lomb's "Unicum " and other similar between-lens shutters, in general use (fig. 65). This necessitates the fitting of the lens to the shutter, but with adapters FIG. 65. - Bausch and Lomb's FIG. 66. - Lancaster's " Unicum " Shutter. " See-Saw " Shutter.

it is possible to fit other lenses. Some forms are, however, suitable for use in front of the lens, such as the " Constant " and Lancaster's " See-Saw " (fig. 66), while those of the double roller-blind type can be used either in front of or behind the lens, though this position is not a favourable one. In these the rectangular form of aperture is the best, circular apertures cutting off a good deal of light, as in the case of drop shutters. W. B. Coventry (op. cit. p. 60) has discussed the action of the double roller-blind shutter as typical of the central class of shutters, and shows that while, under similar conditions, with the lateral shutter the effective exposure is constant and the actual exposure variable at all apertures, it is the reverse with the central shutters, and it will not be so easy to calculate exposures with different sized stops. A central shutter, acting as a diaphragm of variable aperture, gives a more efficient exposure than a lateral shutter of the same dimensions, as long as the opening is greater than the lens aperture, the coefficient of illumination of the lens varying as long as the shutter opening is smaller than that of the diaphragm used. It is desirable, therefore, to increase the speed and use as large an aperture as possible, so that the diaphragm used may be entirely uncovered during the greater part of the exposure.

8. Iris Shutters. - These are a further development of the double curved blade central shutters, and constructed on the principle of the " Iris " diaphragm, with several leaves opening out from the centre of the lens and closing again. They are usually fitted between the lenses of double objectives, and can be made very light and compact. Theoretically. this central position of the shutter is the best, and the " Iris " is the best form for ensuring the most equal distribution of light over the plate, provided, as before, that the opening is equal to the full aperture of the lens. They are made so that the periods of opening and closing may be as short as possible compared with that of full opening. They require great care in construction and fitting to the lens, and so are expensive. They can, however, be used with convertible sets of lenses of different foci fitting the same mount. Several forms are made by British and foreign makers, with three, four or more leaves. Goerz's " Sector " shutter (fig. 67) may be taken as a type. Georgen's " Central " shutter is very light and smooth in working, and can be used in front of a lens for telephoto work. Further details regarding the different forms of shutters, theoretical and practical, will be found in the works by Abney, Coventry, Eder, Fabre and Londe.

Exposure Meters. When gelatin dry plates came into general use, and were made of many different degrees of sensitiveness, the want of a guide to the proper exposure for the various makes of plates under different conditions of lighting began to be felt, and several methods were devised for meeting it. Some of them depend solely upon data derived from observations of the action of the principal factors affecting the result, namely: (I) the speed of the plate; (2) the actinic power of the sun's light for the time of year in a given latitude and its position at the particular time of day: (3) the effective diaphragm aperture of the lens; (4) the nature of the subject and its illumination as affected by local and atmospheric conditions. With others these data are supplemented by, and practically based upon, actinometric observations of the action of the light upon sensitive paper exposed near the camera or the subject at the time. Both methods are in many cases of undoubted use, but the information given by instruments of this kind can only be considered as approximate, and much is left to the judgment of the operator, whose surest guide will be an intelligent study of the principles on which these instruments are based, together with carefully-recorded observations of the combined working of his lenses, shutters, plates and methods of development under the varying conditions of practical work. Before using any of these instruments it is necessary to know approximately the relative sensitiveness or " speed " of the plate in use. In the early days of gelatin dry plates their rapidities were stated as so many times those of wet plates, or (as they are still) " ordinary," " instantaneous," " rapid " or " extra-rapid," terms which, though suitable for one make of plate, may not be so for others. This was improved upon by the adoption, in 1878, of Leon Warnerke's " Sensitometer," which was in use as a standard for some years. It consisted of a transparent scale of 25 squares of different intensities, marked with opaque numbers and arranged so that each third number indicated a doubled rapidity. This was placed in a frame in front of the sensitive plate, and exposed for thirty seconds to the constant light emitted by a phosphorescent tablet, supplied with the instrument, which was previously excited by burning oi.e inch of magnesium ribbon in front of it. The exposed plate was then developed and fixed, and the highest number visible indicated the rapidity of the plate. In 1890 F. Hurter and V. C. Driffield introduced an entirely new system of calculating the sensitiveness of plates of different rapidities. They make a series of exposures in seconds on different parts of the plate in geometrical progression with a standard candle at one metre distance. After development for a certain fixed period with a standard developer, fixing, washing and drying, the " densities " or logarithms of the opacities of the different parts are measured by a special photometer and plotted on a skeleton diagram, producing a curve, one portion of which will practically be a straight line. The position of this line with reference to a scale of exposures given on the diagram decides the rapidity of the plate, while its length indicates the " capacity " of the plate for the truthful rendering of tone. The elaborate investigations by which these results were obtained are of great interest, and were published in the Journal of the Society for Chemical Industry (1890, 1891), and later ones in the Photographic Journal (1898). A complete account of the system by V. C. Driffield was published in 1903, as No. 56 of the " Photo Miniature Series." The sensitiveness shown on the H. & D. scale is directly proportional to the speed number given. The method has been adopted by several dry-plate makers in FIG. 67. - Goerz's " Sector" Shutter.

denoting the sensitiveness of their different brands, and is more or less the basis on which the plate-speeds for the modern English dry-plate actinometers and exposure meters are calculated. Several systems of photometry and measurement of the speeds of dry plates have been discussed at the meetings of the Congres International de Photographie, in 1889, 1891, 1900 and 1905, but no definite standard has been finally adopted. In Germany the use of J. Schemer's sensitometer has been adopted, and appears to be extending. It is based on a system of photographing the graduated tints given by rotating sectors. A full account of the instrument, and of a system of sensitometry based on its use, is given by J. M. Eder in the Photographische Correspondenz 1(1898) p. 469, and (1900) p. 244. In 1901 Chapman Jones brought out a convenient plate-tester on the same principle as the Warnerke sensitometer, but extended by the addition of a colour sensitometer, which is useful for the comparison of orthochromatic dry plates, colour screens, light filters, &c. It consists of a screen plate, 44 X31 in., containing a series of twenty-five tints of graduated densities; a series of coloured squares, blue, green, yellow and red, and a strip of neutral grey, all five being of approximately equal luminosity; a series of four squares of special pure colours, each representing a definite portion of the spectrum; also a space of line design, over which is superposed a half-tone negative. To use the instrument, a quarter-plate of the brand to be tested is exposed behind the screen for a few seconds to the light of a standard candle placed at the distance of a foot, developed, fixed and washed. An examination of the plate will show the sensitiveness, range of gradation, possible range of exposure, sensitiveness to colour, size of grain, amount of halation, "and the most suitable light for development. It can be used for many other tests, and enables any brand of plates to be readily tested by the user and compared with any standard he may find convenient. In making these and similar tests, a standard developer should be allowed to act for a fixed period and at a uniform temperature (Ph. Journ., 1901, 25, P 246)The next important factor is the actinic power of the light. It depends normally on the height of the sun for the latitude of the place at the time when the photograph is taken, and exposures in bright sunlight are found to vary approximately as the cosecant of the sun's altitude above the horizon. The light of the sun itself is practically the same at any given time and place year after year, but is liable to more or less local and temporary diminution by the amount of cloud, haze, dust, &c., present in the atmosphere at the time. It is also affected by the time of day, increasing from sunrise to noon, and then decreasing to sunset. The remaining factor is the effective diaphragm aperture of the lens in relation to its focal length. In most cases of ordinary outdoor exposures this can be taken at its normal value, but becomes smaller and increases exposure if the focal length is much increased for photographing near objects. Besides these principal factors, the nature and colour of the objects, their distance, and the amount of light received and reflected by them under various atmospheric conditions, have a great influence on the exposure required. W. B. Coventry has shown (op. cit. p. 75) how the " light coefficient L," for full sunlight, can be found, and has given a table of values of L for the latitude of London for every hour of the day in periods of ten days throughout the year, also the relative coefficients for " diffused light," " cloudy," " dull " and " very dull." Tables of exposures for different subjects under varying conditions of light have been published by W. K. Burton, A. S. Platts, F. W. Mills, Sir D. Salomons and others, and in preparing them Dr J. A. Scott's tables, showing monthly and daily variations of light for countries about N. lat. 53°, are generally used. The more modern tables, such as are published in the printed " exposure notebooks," also take into account the plate speeds, but unfortunately there is no uniform standard of plate speeds, owing to the difficulty of fixing a definite standard of light. The subject is fully treated in the British Journal Almanac (1901), p. 675, the Watkins Manual, H. Boursault's Calcul du temps de pose en photographie, and similar works by A. de la Baume Pluvinel, G. de C. d'Espinassoux and others.

Based on the same principle as these exposure tables, various portable exposure meters have been brought out, in which scales representing the coefficients for plate-speed, light and diaphragm are arranged as in a slide rule, so that, when properly set, the normal exposure required can be found by inspection, and increased or diminished according to circumstances. In Hurter and Driffield's " Actinograph " the light coefficient is given by a printed card showing the curves for every day in the year and for every hour of the day, the unit being the ih part of the brightest possible diffused daylight when the altitude of the sun is 90°. The " lens " scale shows the ratios of aperture to focal length in general use, and is calculated for single, double and triple systems of lenses. The " speed " scale is based on the exposure in seconds which with one actinograph degree of light will produce a perfect negative of an ordinary landscape. An additional scale is given for five different degrees of illumination - " very bright," " bright," " " " " mean, u very dull." A table of factors for views," " portraiture," " interiors," " copying," is also given, and these regulate the figure to be taken for the exposure. The scales are engraved on boxwood, and there are two sliding pieces (fig. 68).

It is specially adapted for use with plates of speed numbers agreeing with the H. & D. scale, but can be used with any plate of which the relative speed number is known. Convenient exposure meters have been made since 1890 by A. Watkins, of Hereford, in different FIG. 68. - Hurter & Driffield's Actinograph. forms based upon an actinometrical test of the light at the time of exposure. In the complete " Standard Meter " (1890) scales corre- << " << " " sponding to speed of plate," diaphragm numbers," light, " subject " and " enlarging," marked P. D. A. S. and E., are arranged on rings adjustable round a cylinder. The plate-speeds are taken from a table and the " light coefficient," or "actinometer number," is ascertained at the time by exposing a piece of sensitive paper in the actinometer at the end of the instrument for the number of seconds required to match a fixed tint as shown by an attached pendulum. Many improvements have been made in it and the latest pattern (1908) is made in magnalium (fig. 69). The " Dial " meter (1901) is a simpler form in a circular metal case with four apertures marked " plate," " stop," " act " and " exp." above the corresponding scales, and an actinometer for testing the light. The numbers showing the speed of the plate in use, the f value of the diaphragm, and the actinometer exposure in seconds are brought into the respective apertures and the exposure required is read off in the " exposure " aperture. An " indoor meter " is also made, and a " hand camera calculator " for use with the " Standard " or " Bee " meters. The " Queen Bee " and " Bee " meters (1903) are later, smaller and more convenient patterns which have superseded the " Dial " meter and have the plate numbers and exposures marked round the case, and the scales of "f numbers " and " light " on a revolving glass plate. This is revolved till the f number on the right is opposite the speed number of the plate; opposite the " actinometer number " on the left, found as above, will be found the exposure in seconds (fig. 70). The " Queen Bee " meter is similar to the ' ` Bee," but of better construction and fitted with a pendulum.

G. F. Wynne's " Infallible " exposure meter (1893) is also in dial form, but the sensitive paper is exposed directly, no pendulum is used,' and the scales are open on the dial. In use, the glass carrying the movable scale is turned until the actinometer time in seconds upon the exposure scale is opposite the diaphragm number of the plate, as given in the list of plate speeds; the correct exposure will then be found against each stop given on the scale. There are practically only two scales: the scale of diaphragms representing the diaphragm apertures or f numbers, the speed of plate and the variation of exposure due to subject; and the time scale, representFIG. 71. - Wynne's " Infaling the actinometer time and the lible " Exposure Meter. exposure (fig. 71). The actinometer is protected by a yellow glass screen when not in use. In a smaller form the scales are on the 1 Pm A C c?ii a 5: z, FIG. 69. - Watkins's " Standard " Meter.

FIG. 70. - The Watkins's " Bee " Meter.

circumference of a locket, and the actinometer at the back. An " Infallible " Printmeter is also made for showing exposures in contact printing on sensitive papers, but can also be used for testing speeds of plates and papers. Beck's ' ` Zambex " Exposure Meter gives the exposure and stop to be used, also the depth of focus to be obtained with different diaphragm apertures. The required exposure is set to the " speed " number on the next scale of the meter. The third scar corresponds to the times of darkening the sensitive paper in the actinometer attached to the meter, and shows the diaphragm aperture suitable for the given exposure. Other scales show the distances that will be in focus with the different stops used, arranged so that the focal depth of four different lenses can be found. Several other exposure meters are made on the principle of the slide rule, with scale corresponding to the factors of " plate speed," " diaphragm number," " light," " subject," " exposure," and the exposure is found by simple inspection without an actinometer. They are designed for use with particular brands of plates, but can be used for others of similar speeds.

Another class of exposure meters comprises those in which the intensity of the light is estimated visually by extinction through a semi-transparent medium of increasing intensity, such as J. Decoudun's (1888), in which the exposure is judged by the disappearance of a series of small clear openings on a graduated scale of densities when laid on the most important part of the image as seen on the ground-glass. Its indications are not very definite, and the paper scale changes in density after a time. A better form is " E. Degen's Normal Photometer " (1903), consisting of two sliding violet glass prisms, one adjusted for the diaphragm apertures, the other for the actinic illumination of the object. They are mounted with their outer faces parallel. In use the upper slide with prism is drawn out so that the pointer coincides with the division indicating the diaphragm aperture to be used; the object to be photographed is then viewed directly through openings at one end of the instrument, and the lower slide is drawn out and pushed back slowly till the object viewed is almost obscured. The attached pointer will then indicate the exposure required, or, reversing the order, the diaphragm aperture for a given exposure can be found. Auxiliary scales are attached for very short or very long exposures. The principle of construction is that the logarithms of the times of exposure are proportional to the thickness of the coloured prisms. " G. Heyde's Actino-Photometer " (1906) is on a somewhat similar principle, and consists of a circular metal box with dark violet glass viewing screens in the centre of both sides, with an obscuring iris inside the case worked by revolving the back of the box. On the front of the instrument exposure tables are given for plates of every rapidity, and for diaphragm apertures from f/3 to f/45. Exposure meters of this type are specially applicable for open-air work where there is sufficient light for ready measurement. Other simple actinometers are in use for carbon and process printing, consisting generally of translucent graduated scales in different densities of paper, coloured gelatin, &c., or of a photographed scale graduated by increasing exposures. The " Burton actinometer," for pigment printing, made on this principle, contains several small negatives of different densities, one of which is selected of equal depth to the one to be printed, and the progress of the printing is estimated by exposing a piece of sensitive paper under it and examining it from time to time.

Sensitive Plates, Films And Papers Sensitive Dry Plates. - A special feature of modern photography is the use of trustworthy ready-prepared sensitive dry plates and films in different grades of sensitiveness, so that there is no necessity for the photographer to prepare his own plates, nor, indeed, could he do so with any advantage. The practice of outdoor and studio photography has thus been very greatly simplified; and although with wet collodion there was the advantage of seeing the results at once and retaking a picture if necessary, the uncertainties connected with the use of the silver bath and collodion, and the amount of cumbrous apparatus necessary for preparing and developing the plates, far outweighed it. There is also an enormous saving of time, in using dry plates as compared with wet, by deferring development. In tropical climates, also, dry plates can be used when work with wet plates would be impossible. On the other hand, the uncertainty of more or less random exposures on readyprepared p lates must not be overlooked. Besides their use in taking negatives, gelatin dry plates are also largely used for printing transparencies, lantern slides, enlargements, &c. For negative work they are prepared with an emulsion in gelatin of silver bromide, alone or with the addition of silver iodide or chloride, and are to be obtained in five or six degrees of rapidity: " slow," for photomechanical or " process " work; " ordinary," for general purposes when quick exposures are not required; " rapid," for landscape and portraits; " extra rapid," for instantaneous exposures; and " double extra rapid," for very quick snapshot work in dull weather or for special subjects. These latter kinds are exceedingly sensitive, and require great care in use to avoid fog. In order to prevent halation, or irregular action by reflection from the back surface of the glass, dry plates are coated with a non-actinic " backing," which can easily be removed before development.

Self-developing dry plates were introduced in 1906, in which the developing agent is mixed in the film itself, as in the Ilford " Amauto " plate, which only requires immersion in a solution of washing soda for development, or, as in the Wellington " Watalu " plates, applied on the back of the plate, plain water only being required for development, this application also preventing halation. The slow plates used for printing lantern slides and transparencies are usually prepared with an emulsion of silver chloride with or without free silver nitrate and other haloids.

The rendering of photographic plates isochromatic or sensitive to all colours by dyeing them with eosin, or other suitable dyes, has been greatly improved by the use of new dyes, especially those of the isocyanin group, prepared by Dr E. Konig of the Hoechst factory, and known as " orthochrom T," " dicyanin," " pinaverdol," " pinachrom " and " pinacyanol," the latter of which can confer on a silver bromide plate as high a degree of sensitiveness for red as erythrosin does for yellow; also F. Bayer's " Homocol," Dr A. Miethe's " ethyl red," and other similar dyes (see E. Jb., 1905, pp. 18 3, 33 6). Panchromatic plates are now largely manufactured and used for all photographic work in which a true rendering of the relative colour luminosities is essential, and more particularly for the various methods of colour reproduction in which plates are required to be sensitive to red, green and blueviolet. They are made in different degrees of general and colour sensitiveness, according to the purpose for which they are required, the ordinary "isochromatic " being most sensitive for yellow and green, and the " panchromatic " for red, orange and yellow, as well as for green, blue and violet. To obtain the best results from all these plates it is necessary to screen off the blue and violet rays with yellow or orange transparent screens, or colour filters, made of coloured glass, or glass coated with coloured gelatin, collodion, &c., or with glass cells containing solutions of suitable dyes or salts. For the various processes of three-colour reproduction panchromatic plates and special red, green and blue-violet filters have to be used for taking the three negatives, their intensities and absorptions being carefully adjusted to the particular plates in use; the same applies, but less strictly, to the yellow screens used with ordinary isochromatic plates. Dyes specially suitable for these colour-filters have been prepared by Dr E. Konig. Various kinds of colour screens for ordinary, microscopic and trichromatic work are made commercially, and Messrs Schott of Jena make a special yellow glass in three tints for the purpose.

In 1868 Louis Ducos du Hauron, among various trichromatic methods patented for photographically reproducing coloured objects in the colours of nature, described one in which the trichromatic principle, instead of being carried out on three separate plates, was to be combined in one plate by means of a transparent medium covered by a trichromatic screen divided into narrow juxtaposed lines or minute spaces, corresponding to the three primary colours, red, green and blue-violet, the transparent colour of each of these lines or spaces acting as a colour filter. A sensitive panchromatic plate was to be exposed in contact with this screen to produce a negative with lines or spots corresponding to the relative strength of the three coloured lights passing through it, so that a diapositive print on glass properly registered with the tricolour screen would show the object in its proper colours. This method could not be carried out successfully for want of efficient panchromatic plates and other difficulties.

Between 1892 and 1898 several patents were taken out by J. W. McDonough and J. Joly for various methods of preparing trichromatic ruled screens (Ph. Journ., 1900, p. 191). The Joly method was fairly successful in action, but had several disadvantages owing to the coarseness of the lines, the necessity for having two screens, one for taking and another for viewing, and the cost of making them (B. J. A., 1899, p. 671). The " Florence " chromatic plate (1905), worked out in America by J. H. Powrie and Florence M. Warner, was an improvement on the Joly method, the colour screen being photographically printed on a glass plate, coated with panchromatic emulsion and exposed to the coloured object through the screen (Penrose Pictorial Annual, 1905-1906, p. III). Some good results were produced, but it has not come into use.

After several years of laborious research, Messrs Lumiere, of Lyons, adopting Ducos du Hauron's coloured grain method, succeeded where he had failed, and in 1907 brought out their " Autochrome " plates, in a very complete and practical form, making it possible to produce photographs in the colour of natural objects by one exposure instead of three, as in the ordinary three-colour processes. Glass plates are coated with an adhesive medium over which is spread a mixture of potato starch grains, of microscopic fineness, stained violet, green and orange, the interstices being filled in with fine carbon powder to form a tricolour screen, dark by reflected and of a pinkish, pearly appearance by transmitted light. This is varnished and coated with a thin sensitive panchromatic emulsion of gelatino-silver bromide. The plates are exposed in the camera from the back, through the tricolour films, using also a special compensating orange-yellow screen, before or behind the lens, then developed as usual, producing a negative coloured image in the complementary colours, which is then treated and reversed so as to produce a positive coloured image by transmission, showing the picture in its proper colours. The results thus obtained are remarkably good and practically solve the problem of direct colour photography in a simple and fairly inexpensive manner (see Agenda Lumiere, 1909).

In C. L. Finlay's " Thames " colour plate (1908) the tricolour screen is formed by rows of circular dots coloured alternately orange-red and green and the intermediate spaces blue. It is used alone, the coated surface being placed in contact with a panchromatic plate, the uncoated side towards the lens. It carries register marks for adjusting it to the finished picture after development and reversal of the image. These screens, being more transparent than the "Autochrome," require less exposure, but the colour rendering is not so perfect. In the Jougla " Omnicolore " plate (1909) the tricolour screen and sensitive surface are combined on one plate as in the "Autochrome," but the screen is made up of a series of blue-violet parallel lines, with intermediate alternate broken lines of orange-red and yellowish-green at right angles to them, the red narrower than the green. The relative sizes of the coloured dots in the three plates are approximately " Autochrome " starch grains. s o o to sibin.

" Thames " plate, dots, diameter. s ?

" Omnicolore " plates, blue line red square ..

E. Fenske's " Aurora " plate (1909) is a tricolour screen formed by coating a glass plate with a mixture of finely divided particles of gelatin, dyed orange-red, green and blue-violet, without any intervening spaces. The grain generally is coarser and more irregular than in the " Autochrome " plates, but optically corresponds more closely to them than the " Thames " or " Omnicolore " screens do. These plates are issued uncoated for use with any suitable panchromatic plate. A later process is due to Dufay. With the exception of the " Autochrome," these processes are still more or less in the experimental stage.

In order to avoid the weight of glass plates, which may become burdensome on a tour, and also the risk of breakage of valuable records, thin films or sheets of celluloid coated with sensitive emulsions can be used, with great saving of bulk and weight and no loss of efficiency, though such films are sometimes liable to deterioration by long keeping before or after exposure. They are made in two thicknesses, stiff or flexible, the stiff being used exactly as plates, but held in a carrier or simply backed with a card or glass plate, while the flexible are made up in separate sheaths with cardboard backing, as in the " Kodoid " films, or in convenient packages of twelve or more in " film packs " of various patterns. Flexible films of this kind on celluloid have for many years past also been prepared in long strips of different widths suitable for use in hand cameras of the Kodak types and in roll-holders. In the early forms of roll-holders the films were used alone, and being unprotected had to be changed in the dark room, but, as already stated, they are now supplied on spools in cartridges which can be changed in daylight. C. Silvy seems to have been the first to employ this method in 1870. In these cartridges the film is attached to a much longer strip of black paper, and rolled up with it, so that several turns of the paper have to be unrolled before the film is ready for exposure, this point being marked on the outside paper for the successive exposures, with numbers visible through a red screen at the back of the holder. When all have been exposed, the black paper is rolled on for several turns, and when taken out of the holder the loose end is fastened up till the film is developed. As these films are principally used for landscape work, it is now usual to make them isochromatic, and they may be used with or without a yellow screen. They are also made " non-curling " by being coated with gelatin on both sides. Negatives taken on these thin films have the advantage that they can be printed from either side without perceptible loss of definition, which is useful in printing by the single transfer carbon process, and in some of the photo-mechanical printing methods. Flexible transparent films in sheets and rolls have also been prepared upon hardened gelatin, but it is difficult to retain the original dimensions of the film owing to expansion of the gelatin. Paper coated with sensitive emulsions has been successfully used for making negatives in the same way as the celluloid films, and is cheaper, but much more liable to deterioration from atmospheric action before and after exposure, and unless developed soon after exposure the impressed images may fade and become undevelopable. Such papers are, however, still used in meteorological and other self-recording instruments. Stripping films of thin celluloid upon a paper support were introduced by Messrs Wellington and Ward, and had advantages for printing from either side, but are not now made.

Pari passe with the supply of ready-prepared plates, all kinds of photographic printing papers can now be obtained ready for use, so that the photographer has nothing to do with the preparation of his sensitive plates or papers. The old albuminized papers have been generally superseded by ready-prepared sensitive papers coated by machinery with emulsions of silver haloids in gelatin, with or without citrate or other organic silver salts, the chloride being used for most of the " P.O.P." or " printing out papers," which contain more or less free silver nitrate, and in the " self-toning " papers some salt of gold. Some of these printing out papers are also made with emulsions of silver chloride in collodion, and known as " C.C." or " collodiochloride." The basis of most of the developable bromide papers used for en-largements and direct copying, containing no free silver nitrate, and with which an invisible image is brought out by development, much in the same way as with dry plates, is silver bromide. These papers are made in great variety of tints and surfaces, " smooth " and " rough," " glossy " and " matt," for producing different effects. They are largely used for direct printing by artificial light or daylight, for enlargements, and for printing photographic post-cards, &c., in large numbers by machinery, the prints being made on a long band with an almost instantaneous exposure, and developed and fixed by being passed through the proper solutions on large rollers or otherwise. Papers for the platinotype processes, sensitized with salts of platinum and iron, are also manufactured for printing out entirely or for development with potassic oxalate. Prints on these papers have the advantage of being permanent.

Messrs York Schwartz and J. Mallabar's process of developing and toning prints made on a special sensitive paper prepared with an emulsion of silver phosphate was introduced by Messrs Houghton in 1908 under the name of " Ensyna." Very short exposures to day or artificial light are required, and with a special developer (" Ensynoid ") permanent prints are obtained with a varied scale of tones similar to those given by toning with gold, the colour of the print being determined by the exposure, short exposures giving purple and long exposures brown or reddish tones. The process is a rapid one, the operations of printing, developing, fixing and washing being completed within about ten minutes or even less.

For the various methods of printing in permanent pigments (" Autotype," &c.) tissues are prepared coated with pigmented gelatin in various colours, and very successful results in colour photography have been obtained by printing from suitable negatives in three colours with specially prepared yellow, blue and pink tissues. Similar papers, prepared with pigmented gum instead of gelatin, are used in the " gum bichromate " process, and " single transfer " papers, coated with plain gelatin, are used in the pigment printing processes to receive the developed print, and are also useful for photo-lithography, the new " oil-printing " methods, and in trichromatic printing on paper by the Sanger-Shepherd method and Dr Konig's " Pinatype." For Manly's " Ozotype " and " Ozobrome " processes special gelatinized and pigmented papers are made. " Cyanotype " and ' ` Ferrogallic " papers are prepared for the use of architects, engineers, &c., in rolls of considerable width, for the direct reproduction of tracings and drawings as blue or black prints by these and similar methods.

The recognition of the fact that the two principal factors in the development of modern photographic dry plates with a suitable developer are time and temperature, and also that a prolonged immersion in dilute solutions is in many cases a more convenient and equally efficient method of development, has led to the construction of apparatus for enabling the operation to be carried out almost automatically and for timing its duration.

In 1894 A. Watkins brought out his factorial system of development based on the principle " that with a correct exposure on a given plate with a given developing agent, the time of development required for a given printing opacity has a fixed arithmetical ratio to the time of appearance of the high lights of the image, provided the developing power of the solution remains constant during development; and this rule holds good for all variations of strength, amount of alkali or bromide, and temperature within those limits which have been found safe in practice " (Photo. News, 18 94, 38, pp. 115, 729; and further, Ph. Journ., 1900, 24, p. 221). By a series of observations he ascertained the multiplying factors of most of the developers in ordinary use, and in 1905 brought out his " factorial calculator " and a " dark-room clock " for facilitating the working of the method. The former is made of aluminium, and consists of two circular disks, the upper smaller one rotating and carrying a pointer. The outer disk is marked with a scale of Watkins' factors for the different developers, as given in the " instructions " accompanying the instrument, and is used to denote the " time of development " in minutes. The scale on the inner FIG. 72. - Watkins's Factorial FIG. 73. - Watkins's Dark Calculator. room Clock. disk shows the " time of appearance " in seconds or minutes. In use the pointer is set to the factor for the developer in use, and against the " time of appearance " on the inner scale will be found the total number of minutes required for complete development (fig. 72).

The " calculator " can be used with any ordinary clock or watch, but the " dark-room clock " (fig. 73) has been specially constructed for the factorial system. It is an improvement on the earlier forms of Watkins' " Eikronometer," and has a 4 in. dial with io minute and Too seconds divisions, very plain for dark rooms, centre seconds hand, stop action and outside indicator to mark the completed time. The seconds hand completes the revolution in Too seconds, while the minute hand does so in 10 minutes, or sufficient for the longest ordinary development, though it runs on, if necessary, very much longer, both hands starting together always at O.

In 1908 Watkins brought out another system of " thermodevelopment " by time dependent on the use of a standard " time developer," the duration of the development, at a given temperature, being modified according to the make and speed of the particular plate in use. The temperature variations are indicated by a movable scale, or " thermo-calculator," on the bottle of developer, the variations for development speed of various plates being given approximately on the " Watkins' Plate Speed List," which thus shows the " speed of plate " and " speed of development " with the standard developer at 60°. This method is well adapted for plates, films and stand development in tanks or machines, no observation of the plate being required, and the times are most conveniently observed with the " dark-room clock." Full details of these two distinct methods of development will be found in the 4th edition of the Watkins' Manual of Exposure and Development. C. W. Piper's " photographer's stop clock " (1906) is a more elaborate clock, intended for use not only in " time development " but for all photographic operations in which accurate control in regard to time is of importance. It is fitted with a gong and arranged to work by " time " or " bulb." Once started, by pressure on a lever or on the bulb, it will continue to go until stopped, striking the gong at the completion of every minute, when the seconds hand reaches the zero point. A second pressure on the bulb stops the clock, so long as the pressure is continued, while pressure on a lever stops it permanently. It is thus useful for timing any intermittent operations, whilst the clock adds up the separate times and prevents the occurrence of errors difficult to avoid when timing with an ordinary watch. By an additional attachment a prolonged time exposure with the camera may be terminated, or an " instantaneous " or short " time " exposure given at any prearranged time. Messrs Houghton's " Ensign " clock for time development has a dial with 60 divisions, a single hand, and is fitted with a gong. It can be set to ring an alarm bell at the expiration of any period from one minute to one hour, can be started or stopped immediately and is easily read in the dark-room. It requires no winding up, the action of setting providing the tension for the recording movements. It can be stopped and started at will and the bell arranged to give a short or prolonged ring. S. Stanley's is another convenient form, with a 42 in. dial, divided into 60 seconds and 60 minutes, the thick hand recording the seconds and the thin hand the minutes.

Several forms of developing tanks and machines have been constructed for developing a number of exposed plates, together with ordinary or dilute developers, with the aid of the factorial system or independently of it. The Kodak " Automatic Developing Tank " (1905) is a useful arrangement by which bands of exposed roll films can be developed in daylight, without any need of a dark-room (fig. 74). The exposed film is wound from the spool FIG. J4. - Kodak Developing Tank.

into a red celluloid apron contained in a box A, then placed in the tank B, where it is left in a dilute developer for about twenty minutes, and requires no attention. It gives very good results. For the " Brownie " films a special daylight developing box is made. With the Kodak " Eastman Plate-developing Tank " (1908) the exposed plates are removed, in the dark-room, from the plate holders and placed, in pairs back to back, in a special framework holding six pairs, which is lowered into a metal tank containing the developer, and is fitted with a watertight lid so that it can be inverted during development. A clock face, with pointer, by which the period of development may be noted is fitted outside the tank. Another apparatus of the kind is made for developing celluloid films exposed in the " Premo Film Packs " (fig. 75).

Other forms are made, and in some the fixing and washing can also be effected. These tanks undoubtedly save much time and trouble in developing a large number of exposed plates or films, and have been found to work with efficiency and regularity. Messrs Kodak also brought out in 1907 a machine for developing paper prints on bromide or gaslight papers.

FIG. 75. - Premo Film Pack Tank (1908).

Photographic Printing Apparatus For ordinary printing purposes pressure frames, with or without glass fronts, are used for holding the negative and sensitive paper in close contact during exposure to light. They are fitted with hinged backs enabling the progress of the printing to be seen. The pressure is usually given with springs or with screws or wedges acting on the back. They are made in different kinds shown in the dealers' catalogues. For copying large tracings and engineers' drawings by the cyanotype and similar processes large glazed frames are used, mounted on a stand with axle, so that they may be easily turned over for refilling or fixed at a suitable angle to the light. The pressure is given by an elastic cushion or vacuum arrangement, by which air is pumped out from under an indiarubber sheet covering the back of the frame, thus securing a perfectly uniform pressure of about 14 lb to the square inch without strain on the front glass. Such frames are also useful for various photo-mechanical printing processes with large negatives or metal plates.

For rapid printing of post-card and other negatives up to 82 X 61 in. a handy and simple apparatus the " Rapide " has been brought out, consisting of a lantern fitted for oil, gas or electric light, with a sloping front, in which a special printing frame is fixed and arranged so that the prints can be rapidly exposed one after another (B. J. A. 1909), p. 691. In another form arrangements are made for exposing a large number of printing frames on a suitable stand, in one or two tiers round a central arc lamp, which may be provided, as in the " Westminster " revolving printing frame, with a shade to protect the eyes of the operator when examining the prints or changing the frames.

For printing tracings, &c., in long rolls, cylinder and rotatory machines of various types are used, so that the tracing and sensitive paper may be drawn together at a regulated speed in close contact round a glass cylindrical surface within which electric arc or mercury vapour lamps supply the source of light. Several machines of this kind are described in Eder's Jahrbuch for 1908, also in the patent records and photographic journals.

- Apparatus in general: Sir W. de W. Abney, Instruction in Photography (11th ed., 1905); R. C. Bayley, The Complete Photographer (1906); Dr J. M. Eder, Ausfiihrliches Handbuch der Photographic (2nd ed., pt. i. (2), 1892); Jahrbi cher fiir Photographic and Reproductions Technik (E. Jb.), (1887-1908). Valuable for reference on all forms of apparatus: Dr C. Fabre, Traite encyclopedique de photographic (T. E. P.) (vol. i., 1889; Supplements A, 1892; B, 1897; C, 1902; D, 1906), also gives much information about photographic apparatus and optics; Chapman Jones, An Introduction to the Science and Practice of Photography (4th ed., 1904); British Journal Photographic Almanacs to 1909 (B. J. A.); Patent Office, Abridgments of Specifications, class 98, " Photography "; Photography Annuals (1891 to 1899); Photographic Journal (Ph. Journ.); Year Books of Photography to 1907.

Lenses and Optics: C. Beck and A. Andrews, Photographic Lenses (6th ed.); W K. Burton, Optics for Photographers (1891) R. S. Cole. A Treatise on Photographic Optics (1899); T. R. Dallmeyer, Telephotography (1899); J. A. Hodges, Photographic Lenses (1895); Captain Houdaille, Sur une methode d'essai scientifique et pratique des objectifs photographiques (1894); G. L. Johnson, Photographic Optics and Colour Photography (1909); 0. Lummer, Contributions to Photographic Optics, translated and augmented by Professor S. P. Thompson (1900); Dr A. Miethe, Optiq,ce photographique sans divellopements mathimatiques, translation by A. Noaillon and V. Hassreidter (1896); Lieut.-Colonel P. Moessard, L'Optique photographique (1898), L'Objectif photographique (1899) C. W. Piper, A First Book of the Lens (1901); Dr M. von Rohr, Theorie and Geschichte des photographischen Objectivs (1899), a most valuable theoretical and historical summary of photographic optics and its literature; Hans Schmidt, Das Fern-Objectiv im Portrcit, Architectur, and Landschaftfache (1898); Dr H. Schroeder, Die Elemente A der photographischen Optik (1891); J. T. Taylor, The Optics of Photography and Photographic Lenses (3rd ed., 1904); The " PhotoMiniature Series," No. 1 (1899), Modern Lenses, No. 26 (1901), Telephotography; No. 36 (1902), Lens Facts and Helps; No. 79 (1907), The Choice and Use of Photographic Lenses. Hand Cameras, Shutters, Exposure Meters, &c.: Sir W.de W. Abney, Instantaneous Photography (1895); H. Boursault, Calcul du temps de pose en photographie (1896); W. B. Coventry, The Technics of the Hand Camera (1901), the working principles of lenses, shutters, &c., for instantaneous exposures are treated mathematically and practically; L. David, Die Moment-Photographie (1898); G. de Chapel d'Espinassoux, Traite pratique de la determination du temps de pose (1890); Dr R. Kriigener, Die Hand Camera and ihre Anwendung fir die Moment-Photographie (1898); A. Londe, La Photographie instantanee, theorie et pratique (3rd ed., 1897); F. W. Pilditch, Drop-Shutter Photography (1896); A. de la Baume Pluvinel, Le Temps de pose (1890); A. Watkins, The Watkins Manual of Exposure and Development (4th ed., 1908). The Practical Photographer, No. 8 (1904), " Hand Camera Work." .The " Photo-Miniature Series," No. 3 (1899), Hand Camera Work; No. 37 (1902), Film Photography; No. 56 (1903), The Hurter and Drifeld System; No. 76 (1906), The Hand Camera; No. 77 (1907), Focal Plane Photography. Colour Photography: Agenda Lumiere, La Photographie des couleurs et les plaques autochromes (1909); G. E. Brown and C. W. Piper, Colour Photography with the Lumiere Autochrome Plates (1907); Baron A. von Hiibl, Three Colour Photography, translated by H. O. Klein (1904); Theor-ie and Praxis der Farben Photographie mit Autochrom Platten (1908); G. L. Johnson, Photographic Optics and Colour Photography (1909); Dr E. Konig, Natural Colour Photography (trans. by E. J. Wall (1906); Die Autochrom Photographie and die verwandten Dreifarbenaster-verfahren (1908). G. WA.) III.-Pictorial Photography Pictorial photography differs from other branches of photographic practice in the motive by which it is prompted. Employing the same methods and tools, it seeks to use photographic processes as a means of personal artistic expression. Thus in the early days of Fox Talbot's calotype, about 1846, David Octavius Hill, a successful Scottish painter, took up this method of portrayal, and, guided by an artist's knowledge and taste, and unfettered by photographic convention, which indeed had then scarcely begun to grow, produced portraits which for genuine pictorial quality have perhaps never been surpassed, especially if some allowance be made for the necessary imperfections of the " Talbotype " (see plate). Whether they were in their day typical examples of Talbotype with all the latest improvements, Hill probably never cared. When, again, a few years later, Sir William J. Newton, the eminent miniature painter, read a paper before the newly formed Photographic Society of Great Britain (now the Royal Photographic Society), his recommendation to depart from the custom of defining everything with excessive sharpness caused his address to be almost epoch-making. " I do not conceive it to be necessary or desirable," he said, " for an artist to represent, or aim at, the attainment of.: every minute detail, but to endeavour at producing a broad and general effect.. .. I do not consider that the whole of the subject should be what is called ` in focus '; on the contrary, I hate found in many instances that the object is better obtained by the whole subject being a little out of focus." The doctrine has been persistently repeated ever since, but only within the last decade of the 19th century was the suppression or diffusion of focus received by photographers generally with anything better than ridicule or contempt, because it was unorthodox. O. G. Rejlander, Mrs Julia Margaret Cameron, H. P. Robinson, and others, by precept or practice, strove against such photographic conventions as had arisen out of those technical exigencies to which pictorial qualities were so often sacrificed. As late as 1868, in the Manual of Photographic Manipulation, by Lake Price, the old advice to arrange a group of persons in crescent form, so as to adapt the subject to the curve of the field of the lens, was repeated with the additional recommendation of plotting out on the ground beforehand the " curve of the focus " as a guide. As a defiance of this dictum, Rejlander, in 1869, produced a group of the members of the Solar Club in which some of the chief figures were set widely out of the " curve of the focus." The mere technical difficulties of this performance with wet collodion plates, and in an ordinary upper room, need not be touched upon here, but it is to be noted as one of those triumphant departures from convention which have marked the progressive stages of pictorial photography. At about the same period, Mrs Cameron, carrying the recommendation of " a little out of focus " rather further, regardless of how her lens was intended to be used by its maker, secured the rendering dictated by her own taste and judgment, with the result that many of her portraits, such as those of Tennyson, Carlyle, &c., are still in their way unsurpassed. Contemporaneously, Adam Salomon, a talented sculptor, " sunned " down the too garish lights of his photographic prints, and strengthened the high lights by working on the back of the negative.

But, during the concluding quarter of the 19th century, probably the most powerful influence in pictorial photography was that of H. P. Robinson, who died in February 1901, and, but for a brief period about the year 1875, was one of the most prolific " picture makers." Inspired by Rejlander, of whom he was a contemporary, Robinson will perhaps be best remembered by his earlier advocacy of combination printing. As early as 1855 Berwick and Annan exhibited a photograph which was the result of printing from more than one negative, a figure from one plate being cunningly introduced into a landscape print from another. Then came from Rejlander " The Two Ways of Life," in which, with wonderful ingenuity, thirty different negatives were combined. Robinson followed, and between 1858 and 1887 exhibited numerous examples of combination-printing, one of the most popular and fairly typical examples being " Carolling " (see plate), which received a medal in the exhibition of the Royal Photographic Society in 1887.

Though in this combination-printing one may perhaps perceive the germ of incentive towards the production of special effects not seen in the original, yet the practice was not destined to become very popular, for even in the most capable hands there remains the difficulty, if not impossibility, of fitting a portion of one negative into a print from another and still preserving true relative tonality, and even true proportion. Skilfully produced, eminently popular in character though " Carolling " may be, such errors are not absent. Of this combination-printing Dr P. H. Emerson has said: " Cloud printing is the simplest form of combination-printing, and the only one admissible when we are considering artistic work.Rejlander, however, in the early days of photography, tried to make pictures by combinationprinting. This process is really what many of us practised in the nursery, that is, cutting out figures and pasting them into white spaces left for that purpose in the picture-book. With all the care in the world the very best artist living could not do this satisfactorily. Nature is so subtle that it is impossible to do this sort of patchwork and represent lber. Even if the greater truths be registered, the lesser truths, still important, cannot be obtained, and the softness of outline is easily lost. The relation of the figure to the landscape can never be truly represented in this manner, for all subtle modelling of the contour of the figure is lost." Pictorial photography received a large accession of votaries in consequence of the greater,facilities offered by the introduction of the gelatino-bromide, or dry-plate, process, which, although dating from 1880, did not notably affect photographic communities until some years afterwards; and although improvement in appliances and instruments had little to do with the advance of the pictorial side of photography, yet, indirectly at least, the dry-plate and the platinotype printing process have had an undoubted effect. The former gave enormously increased facility, and dispensed with tedious manipulations and chemical knowledge, while its increased light-sensitiveness decreased the limitations as to subjects and effects. The platinotype process was discovered in1874-1880by W. Willis, who employed his chemical skill and knowledge to give the world a printing process more likely than the hitherto prevalent silver papers to satisfy artistic requirements.

Up to 1882 but few outdoor photographers had ventured to run counter to the general dictum that photographs should only be taken during sunshine or good bright light, and unquestioning xxl. 17 a consent would have been given everywhere to the proposition that it would be absurd to work when anything like fog or atmospheric haze was present. Isochromatic plates, introduced for the purpose of equalizing the actinic power of various colour luminosities, and so rendering colours in correct relative value, were recommended by one writer, who applauded their supposed advantage of enabling the photographer to photograph distance without any suggestion of atmosphere. That evening or morning haze might enhance the beauty of a landscape, or that the mystery of half-concealment might itself be beautiful, does not seem to have occurred to the photographer, who had become infatuated by the exquisite clearness and sharpness which, with a minimum of labour, he was able to achieve. It is therefore interesting to note one of the first photographic successes which broke away from this convention, just as Rejlander's Solar Club group defied the formula of arranging human figures like the tiers of an amphitheatre. William M`Leish, of Darlington, a Scottish gardener who had taken to photography, and who seems to have been less under the influence, or it may have been that he was ignorant, of the old dicta, sent to the Royal Photographic Society's Exhibition in 1882 a photograph entitled " Misty Morning on the Wear," a very beautiful view of Durham Cathedral as seen through the mist from across the river. The judges, although they that year awarded eleven medals, passed this by; but appreciation came from outside, for newspaper critics, and practically all those who were not blinded by prejudice and conventionality, declared it to be the photograph of the year. The exhibitions immediately succeeding revealed numerous imitators of M`Leish, and both figure and landscape work began to be shown in which there was evidence of greater freedom and originality.

Meanwhile the Photographic Society of Great Britain had drifted away from its artistic starting-point, and had become chiefly absorbed in purely scientific and technical subjects. But the general apathy which existed in respect of the artistic aspirations of some workers was the forerunner of a period of renaissance which was to end in lifting the pictorial side of photography into a greatly improved position. In 1886 Dr P. H. Emerson read before the Camera Club a paper on " Naturalistic Photography," which served as an introduction to the publication (1887) of his book under that title. Unquestionably this book struck a powerful blow at the many conventionalities which had grown up in the practice of photography; the chief doctrines set forth being the differentiation of focus in different planes, a more complete recognition and truer rendering of " tone," a kind of truthful impressionism derived from a close study and general acquaintance of nature, and a generally higher and more intellectual standard. After the publication of a second edition in 1889 Dr Emerson publicly renounced the views he had published, by issuing in January of 1891 a bitterly worded, black-bordered pamphlet, entitled The Death of Naturalistic Photography. But the thoughts which the book had stirred were not to be stilled by its withdrawal. Towards the end of the same year the conflict which within the Photographic Society had become apparent as between the pictorial enthusiasts and the older school, culminated in connexion with some matters respecting the hanging of certain photographs at the exhibition of that year; and a number of prominent members resigned their membership as a protest against the lack of sympathy and the insufficient manner in which pictorial work was represented and encouraged. This secession was to prove the most important event in the history of that branch of photography. The secessionists being among the most popular contributors to the annual exhibition gathered round them numerous sympathizers. In the following year they formed themselves into a brotherhood called " The Linked Ring," and in 1893 held their first " Photographic Salon," at the Dudley Gallery, Piccadilly. The most noteworthy of the early adherents attracted to the new body was James Craig Annan, whose work was practically unknown until he exhibited it at the first Salon; and almost at once he, by general consent, took a position amongst pictorial photographers second to none (see plate).

Aroused into greater activity by these events, the Royal Photographic Society began to pay more attention to what had now become the more popular phase. At subsequent exhibitions the technical and scientific work was hung separately from the " Art Section," and a separate set of judges was elected for each section. It became the custom to allot by far the greater amount of space to the " artistic "; and later, artists were elected as judges, by way of encouraging those who were devoted to the pictorial side to send in for exhibition. In the autumn of 190o the New Gallery was secured, and a comprehensive exhibition of all phases of photography was held.

It is interesting to note that as a distinct movement pictorial photography is essentially of British origin, and this is shown by the manner in which organized photographic bodies in Vienna, Brussels, Paris, St Petersburg, Florence and other European cities, as well as in Philadelphia, Chicago, &c., following the example of London, held exhibitions on exactly similar lines to those of the London Photographic Salon, and invited known British exhibitors to contribute. The international character of the " Linked Ring " encouraged an interchange of works between British and foreign exhibitors, with the result that the productions of certain French, Austrian and American photographers are perfectly familiar in Great Britain. This, in the year 1900, led to a very remarkable cult calling itself " The New American School," which had a powerful influence on contemporaries in Great Britain.

It may be well to glance at such improvements of process or apparatus as have not been direct and essential means to pictorial advance, but rather modifications and improvements made in response to the requirements of the artistic aspirant. Such improvements are of two orders - those which are devised with the aim of securing greater accuracy of delineation, the correction of distortion and of apparent exaggeration of perspective, and the more truthful rendering of relative values and tones; and those which seek to give the operator greater personal control over the finished result. While great advances have been made in photographic optics, it cannot be said that pictorial work has been thereby materially assisted, some of the most successful exponents preferring to use the simplest form of uncorrected objective, or even to dispense with the lens altogether, choosing rather to employ a minute aperture, technically called a " pinhole." This is but one example of many which might be quoted to bear out the statement that in photography the advance of anything in the nature of artistic qualities has not been correlative with mechanical improvements. The hand camera can only be said to have had an indirect influence: it has increased the photographer's facilities, and by removing the encumbrance of heavy tools has widened his sphere of operations; but it is perhaps in connexion with the plates and printing processes that more direct advantages have been gained. The fact that the actinic power of colours is not proportional to their luminosity was long regretted as an obstacle to correct representation; but by the introduction of orthochromatic or isochromatic plates in 1886 (when B. J. Edwards bought the Tailfer and Clayton patent, under which he shortly brought out his orthochromatic plates) this original disability was removed; while with increased rapidity in the isochromatic plate colour values may still further be corrected by the use of coloured screens or light filters, without interfering with the practicability of making sufficiently rapid exposures for most subjects. Again, by a better knowledge of what is required in artistic representation, certain modifications in the formulated treatment of ordinary and uncorrected plates are found to do much towards removing the evil; hence, with an ordinary plate " backed " so as to counteract overexposure of the higher lights, an exposure may, except in extreme cases, be given of length sufficient to secure the feeble rays of the less actinic colours, and by subsequent suitable development a result hardly distinguishable from that of a colour corrected plate may be secured. Chemical experiment has placed in the photographer's hands improved and easier means of entire, unequal and local intensification and reduction, but utility of these is restricted. By the artistic worker it is claimed that the lens and camera are but the tools, and the negative the preliminary sketch or study, the final print standing to him in the same relation as the finished painting does to the artist. In the production of the print various means of personally controlling the formation of the image have been resorted to. Thus the local development of platinotype by means of glycerine has its champions, but it seems to have been little used, its resuscitation being chiefly due to two or three prominent workers in New York. Here should also be mentioned the revival in 1898 of rough-surface printing papers, chiefly those sensitized with silver, the roughest texture drawing papers being employed to break up the excessive sharpness of the photographic image, and by the superficial inequalities introducing the effect of luminousness to over-dark shadows and variety to blank whites. The almost forgotten process of Pouncy, and of Poitevin, now known as the gum bichromate process, was rehabilitated in 1894 by M. Rouille, Ladeveze expressly to meet the needs of the pictorial worker. Perhaps the best results that have been achieved by it are those of M. Robert Demachy of Paris, though many English workers have used it with remarkable success. In it paper of any kind may be selected as the support. The power of the operator to modify the printed image to almost any extent, even to introducing and eliminating lights and shadows, and in other ways to depart widely from the image given by the negative, depends upon the fact that the coating of gum and pigment (which, being bichromatized, becomes insoluble in proportion as it is acted upon by light) holds the pigment but imperfectly, and yields it up upon a vigorous application of water. According, therefore, to its application or retention, the operator can lighten or deepen in tone any portion. Numberless variations of other methods, such as brush development and local toning or stopping, have been suggested with the same object. Other workers have shown that by dexterously shutting off and admitting the light to various parts of the negative whilst printing, the disposition of the lights and shades in the print can be modified to so great an extent as to alter the general contour of the scene. Examples of an original unaltered print, and one which has been thus modified, are shown in the accompanying plate. Portions are shaded in by allowing the light to have access to the print, either through the negative - in which case the image with all its details, prints more deeply - or by removing the negative, when the action of the light is to flatten and suppress both detail and contrast. Latterly some few have resorted to extensive working on the negative, both on the back and on the film; drawing by hand is practised on the film to render too prominent features less obtrusive, and objects in the background are merged by an intricacy of lines and cross-hatching. Many of the results are very pleasing, although one hesitates to justify the means, however good the end. On the other hand, to exclaim for purity of method and the exclusion of extraneous aids is very like setting up an arbitrary standard no less unreasonable than those conventions against which pictorial photography has so long striven.

Authorities.-P. H. Emerson, Naturalistic Photography; H. P. Robinson, Picture-making by Photography; Art Photography; Pictorial Effect in Photography; Elements of a Pictorial Photograph; A. H. Wall, Artistic Landscape Photography (1896); A. Horsley Hinton, Practical Pictorial Photography (1898), and subsequent editions; C. Puyo, Notes sur la photographic artistique (Paris).

(A. H. H.)