By the way, the student may be interested in knowing that Herr Stenger has worked out the relative exposures with ordinary non-colour-sensitised plates, using, of course, the proper filters. They are, blue 1, green 750, red 9,000. The ultimate results are, he contends, exactly similar to those obtainable on panchromatic plates.

One-Exposure Cameras

Obviously, a system which will give all these exposures simultaneously is to be preferred. But the triple-lens system, as hitherto applied, is unsuitable for the photography of near objects. The images from the three lenses side by side will each represent an image, varying only slightly, perhaps, yet sufficiently to make coincidence impossible with regard to any object nearer than one hundred times the focal length. Herein comes the advantage of the Sanger Shepherd one-lens, one-exposure camera, in which, by means of prisms, the three exposures are given at one time, and relatively through each screen. Only two sizes are at present to be obtained to give negatives 2§ x 2in., and about lantern size; but, of course, it is easy to make enlarged negatives from these on ordinary slow plates. The method relieves the operator from all anxiety, as there is not the slightest danger of varying exposure, or of the register being lost by any movement of the camera before the three negatives have been secured.

Development is not a very serious matter, if care be taken not to work too near the source of light, and to use a movable cover (a cardboard-box lid will do) for the developing dish. The plate becomes less sensitive as development proceeds; great density should be avoided, and evenness of development is important.

Lantern Slides

According to the older fashion, three transparencies in black and white were made from the three negatives, and projected by a triple lantern through red, green and violet glasses. This system is now replaced by the printing of three coloured positives on thin celluloid; and when these are bound together, and projected through the ordinary lantern, they should reproduce with fair accuracy all the colours of the original. Lantern slides projected by cither method, when carefully executed, far surpass in brilliancy anything hitherto effected by means of the screen plates.

Printing On Paper

One important difference must be noted, however, between illuminating transparencies through coloured glasses and colouring the actual positives. A photograph is a record, not of lights, but of shadows. We must therefore employ in printing, not the colours through which the original negatives were taken, but those which are as nearly as possible complementary to them. The complementary colour to the blue-violet is yellow, to green pink, and to red a blue with a very slight tinge of green in it. And it is these colours which we must have recourse to, whether for the Sanger Shepherd slides, or for prints in carbon or gum-bichromate from three-colour negatives.

Reproduction Of Screen Colour Plates

In the studio, where the light sources are under control, and the studio or process cameras provide for exact registrations, most of the difficulties of the landscape artist in producing three-colour negatives are non-existent. A new field has been opened by the advent of the Autochrome, Thames, and similar plates, which require translation into three-colour negatives for reproduction on paper. In practice there is a certain amount of difficulty, owing to the nature of the image, wherein the three synthetic colours lie side by side, instead of being blended into homogeneous masses of colour as in nature, each colour occupying a comparatively small portion of the total area of the plate. A photographic negative, therefore, from either colour, also only occupies a portion of the plate, the intermingled portions, which theoretically should be quite transparent, representing the areas occupied by the other colours.

Mr. Howard Farmer, the well-known Principal of the Polytechnic School of Photography, has explained how this trouble is overcome in his very successful reproductions. The secret consists in imparting to the image just sufficient an amount of diffusion to cause a spreading of the silver deposits on the negatives so that they fill these intermingled spaces. The screen plate is in itself a composition of grained and mottled texture, and will therefore bear without loss of visual definition such devices as the diffusing glass, vibratory motion, irradiation or intentional diffusion of focus. Success will depend upon practice, as the diffusion of the image must be sufficient to fill in the spaces, or the mottled effect will be rather worse than when the film is sharply focussed.