The production of colour photographs by the super position of dyed positives is known as the subtractive method, because the final colours are obtained by the action of the dyes in each subtracting, from the white light falling upon the photograph, the rays that are not required. The Sanger Shepherd process may be taken as typical of the many pro cesses for the production of transparencies by the subtractive method, differing from each other only in detail.
Colour Photographs on Paper.—The subtractive method is also applicable to the production of colour photographs on paper, and many processes have been devised by which this can be successfully accomplished. Ducos du Hauron suggested in 1869 the use of three carbon tissues, pigmented respectively in red, yellow and blue. This suggestion has been successfully put into practice within the last few years, and very beautiful results have been obtained. To facilitate exact superimposition of the prints, the carbon tissues are supplied on temporary supports of transparent celluloid film. The process has been introduced commercially in England by the Rotary Photographic Co. and the Autotype Co., and all the necessary materials are readily obtainable.
In the Lumiere process, introduced by Messrs. Lumiere, of Lyons, a print on glossy bromide paper is made from the red-filter negative, and toned blue. Prints from the other two negatives are then made in bichromated gelatine on a transparent celluloid support, and thes, after being stained yellow and pink respectively as described above, are super imposed on the blue print, each being stripped from its celluloid support after being brought into exact register with the blue print.
Messrs. Sanger Shepherd & Co.'s very successful Imbibition Process is based on the discovery that a thin film of soft gelatine on damped paper, placed in contact with a dyed gelatine relief, will quickly absorb all the dye from the latter. The process consists in printing from the triple negative in bichromated gelatine on a transparent celluloid support. The developed prints are stained in the usual colours, and successively brought into close contact with a sheet of damped gelatine-coated paper, by which the stains are absorbed, the gradations of each positive being exactly represented by the depth of the staining. A complete colour print is thus produced on the paper, and the celluloid positives, entirely cleared of their dyes, are ready for use in making another paper print, and may be used again and again, there being no necessity, as in the processes before described, to make a fresh set of positives for every print. Another advantage of this process is that the three dyed positives may be viewed in superposition over a sheet of white paper, as a test of the colours, before bringing any of them into contact with the printing paper, and any errors in depth of staining may thus be detected and rectified in time to prevent spoiling a print.
" Pinatype," introduced by Dr. Konig in igo5, is another process in which colour prints are produced by transference of dyes to gelatine-coated paper from bichromated gelatine " printing plates." The peculiarity of this process is in the dyeing of the printing plates. In the Sanger Shepherd process it is the hardened gelatine image that is dyed ; the Pinatype dyes, on the contrary, are taken up only by the unhardened gelatine. The printing plates are therefore
made by exposure under positives, instead of direct from the negatives, and do not require development, as the un hardened gelatine must not be removed. As in the Sanger Shepherd process, the printing plates may be used over and over again.
The Screen-Plate Processes.—Excellent as are the results obtainable by the indirect or triple-negative processes al ready described, no method which involves so many opera tions can be regarded as ideal in its simplicity ; and although the invention of these processes made three-colour photo graphy and three-colour printing a commercial possibility, they were too lengthy and troublesome to be adopted by more than a very small minority of amateurs. The long-felt want was a photographic plate by means of which a direct colour photograph could be obtained, with approximately the same ease and certainty as an ordinary photograph in monochrome. This desideratum has been realised in the screen-plate.
The fundamental principle of the screen-plate processes is the same as that of the triple-negative method already described—the separation of the colour rays by means of light filters of red, green and blue-violet. It is in 'the application of the principle that they differ from the earlier method. The screen-plate avoids the necessity of making three separate negatives, by combining the three light-filters in the form of an infinite number of microscopic lines, dots, patches, or grains of the essential colours, evenly distributed over the entire area of the plate. Superimposed upon this composite colour-screen is a film of photographic emulsion, made sensitive to all colours. The plate is exposed with the glass side towards the lens, so that all the rays of light which go to form the picture must first pass through the microscopic light-filters. Naturally, when the plate is developed, silver is only deposited under the red particles of the screen in those places where red rays fell upon them, and those portions of the emulsion covered by the green and blue-violet particles of the screen are only affected in the areas where green and blue rays respectively fell. The result is a composite negative image—the three colour records split up into countless thousands of separate but closely adjacent particles or lines. The colour-screen, be ing protected by a special varnish, remains unaffected by the development of the sensitive film above it, and if the plate is fixed after development and examined by trans mitted light, a negative image is seen in complementary colours, the positive colours being blocked out by the silver deposit. Instead of fixing at this stage, however, the negative image is usually converted into a positive, by dis solving out the reduced silver bromide and redeveloping the plate after exposing it to daylight—two very simple operations, occupying only a few minutes. The effect of the redevelopment is to deposit silver over all those portions of the colour-screen through which the light rays did not pass during the original exposure : thus all particles of colour not required for the finished picture are blocked out ; and as the original silver deposit has been dissolved away, the colours blocked out by the first development are disclosed, the result being a transparency in the natural colours of the objects photographed.