COLOUR PHOTOGRAPHY 864. The Various Processes. Since the begin nings of photography there has been the wish to reproduce not only the shape and the tones, but also the colours of the objects photographed, and a long list could be compiled of rogues who, by pretending to have invented marvellous processes for the photographic reproduction of colours, have been able to rob the artless, who have allowed themselves to be deceived by some tricks of sleight-of-hand.
The first hopes of success were given by the experiments of A. E. Becquerel (1848), who, making use of a phenomenon previously pointed out by Seebeck (r8ro), was able to reproduce the colours of the solar spectrum and then those of some objects on silver plates the surface of which was incompletely chlorided (at that time it was believed that a superficial layer of silver sub-chloride was formed). The results were only approximate, and these images could not be fixed. Nor did more success attend later at tempts by a host of experimenters, who, instead of the silver plate, used a print-out paper previously fogged uniformly to produce a purplish colourt (photo-chloride ; footnote to § Fruitless attempts had been made to utilize an observation made by Chevreul to the effect that the mixture in variable proportions of three suitably chosen pigments enables a large number of colours to be formed. The principles of trichromatic photography were, however, first clearly set forth by J. Clerk Maxwell (1861), who was the first to conceive and use the idea. of selection negatives and three-colour projec tion. In 1867 C. Cros, a writer of humorous fiction, deposited with the French Academy of Sciences a sealed envelope (not opened until 1876) in which he laid down some of the prin ciples of three-colour photography. Following the publication in February, 1869, of a patent applied for in the previous year by L. Ducos du Hauron, Cros published a paper on this subject, but it may be said that the most complete outline and forecast of the many working processes of three-colour photography were given by Ducos du Hauron in 1869. Although the communications of Ducos du Hauron were accompanied by some specimens, the lack of scientific precision in his descriptions, his allu sion.s to an abandoned physiological theory, and the belief, general at that time, in the complete inactivity of green and red light on photographic preparations, all provided material for a violent campaign which deprived the inventor of all support for exploiting his processes and post poned their application until, twenty years later, these same processes were " re-invented." An observation by Grothuss (1819) that fugitive colour substances are destroyed by the radiations complementary to those transmitted by the respective substances led C. Cros (1881) and R. E. Liesegang (1889) to suggest the possibility of reproducing the colours of a trans parent polychrome image by exposing to light, beneath it, a sensitive layer formed of the black mixture of three fugitive colours, red, yellow, and blue. Such bleach-out colour films (extra ordinarily slow) were worked out in particular by E. Vallot (1895), R. Neuhauss (1902), and K. Worel (1902). A very considerable advance in these processes, due to J. H. Smith, enabled a paper (Uto, perfected in 1911 under the name lJtocolor) to be placed on the market which yielded a copy after about an hour's exposure to sunshine, without, however, ensuring the permanence of the image. This process has
been abandoned, although some experimenters are still endeavouring to perfect it.
A method conceived and worked out by G. Lippmann in 1891, as an experimental demon stration of the phenomena of the interference of light, has furnished results of rare beauty and absolute perfection, but for lack of being able to obtain commercially the apparatus and the plates required, and perhaps also because of the exposures being very much longer than those usual at the present time, this process has been little used and has been almost abandoned. A perfectly transparent, grainless, gelatino bromide emulsion is rendered panchromatic and eXposed to light on its glass side in a special slide, forming a hermetically sealed tank of which the plate forms the front, and which is filled with mercury, thus obtaining a mirror in optical contact with the emulsion. The light reflected " interferes " with the incident light, forming within the emulsion a system of alternating light and dark layers, parallel and equidistant. After development, the reduced silver gives rise within the emulsion to a stratified formation of which the laminae, corresponding with the light layers, represent layers of gelatine which have, in the case of elementary colours, a thickness equal to half the wave-length of the incident radiation (it is not possible to discuss here the case of complex colours, which are, however, quite as perfectly reproduced as the elementary ones). These thin laminae have exactly the correct thickness to reproduce the incident colour by reflection (colour of soap-bubbles). 1 The three-colour processes, as practised com mercially, require three negatives to be taken, and then the superposition in accurate register of the three elementary positive images. These operations, while presenting no unsurmountable difficulty, are certainly delicate and slow, and therefore this method is seldom practised except in photo-mechanical processes. One of the methods outlined in 1869 by Ducos du Hauron consisted in intermingling the three elementary images instead of superposing them. A some what crude attempt to apply this idea was made by J. Joly with only very limited success. He used a screen, in each inch of which were i8o bands, alternately violet, green, and orange. This process did not come into general use by professionals and amateurs until the production in 1907 by A. and L. Lumiere of the Autochrome plate (with a trichrome mosaic screen formed of elements of microscopic size).
Starting both from a suggestion by J. Szczepanik (1899) and from the working methods specified by G. Lippmann for integral photo graphy (§ 835), R. Berthon (1908) described a process of three colour photography where the three images are intermingled in the emulsion of a film of which the naked surface has been embossed to form lens elements of microscopic size. This process has been exploited in 1923 for professional cinematography by Keller Dorian, and later for sub-standard amateur cinematography by the Kodak Company (1928), and, subsequently, by various other firms.
While it is necessary always to be exceedingly cautious in denying future possibilities, it does not seem rash to affirm that any inventor announcing that he has succeeded in reconsti tuting the colours of a subject by means of an ordinary photographic negative exposed under whatever conditions, must be either a lunatic, a humbug, or a rogue.