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374 HISTORY OF COLOR PHOTOGRAPHY
and Andresen, failed to notice that these solutions could be used as developers until after Fischer made his disclosures and together with Siegrist published a full account of the chemistry and properties of such solutions. In 1914, a year after the fact was made known generally, Vidal made the discovery that his solutions also had developing power (Fr. P. 468537). Instead of using pure dichromate as the sensitive medium, A. Thiebau (La Phot. (1908), p. 227; Brit. J. Phot., Vol. 55 (1908), p. 738), suggested the following as being more sensitive
Ammonium dichromate 90 parts
Copper sulphate 45 parts
Manganese sulphate 10 parts
Water to 1000 parts
After exposure, the paper was washed, then immersed in a 5 per cent solution of one of the following agents, slightly acidulated with sulphuric acid.
Aniline hydrochloride
Phenols
Pyrogallic acid
Cresols
Naphthols
Para-amino-phenol
Para-phenylene-diamine
etc.
In 1912 Dr. Rudolph Fischer disclosed the fact that the latent image in a silver halide emulsion could replace the reduced chromium oxide images utilized by Kopp, Gusserow and Andresen, Vidal, Thiebau, etc. Whether or not he was familiar with the work of these people is a question open to debate, for neither in his basic patents (Ger. P. 253335; Eng. P. 2562/13, 5602/13; U.S.P. 1079756 and 1102028) nor in the article which he wrote together with Siegrist (Phot. Korr., Vol. 51 (1914), p. 18) does he mention it. But he does discuss the disclosures of Homolka on primary color development (cf. preceding chapter). The article covers the fundamentals so thoroughly that it well merits the detailed abstraction which follows. It is entitled "The Preparation of Dyes by Oxidation by Means of the Latent Image."
This subject has received but scant attention because it was very difficult to isolate and purify the dye formed. But the phenomenon is well known. The brown stain of a pyro-developed image is due to an oxidation product, and this allows itself to be isolated from the silver. In 1907 Homolka showed that indoxyl and thio-indoxyl would develop a latent image and yield the dyes indigo and thio-indigo together with the silver. This brings to mind the agents para-amino-phenol, and para-phenylene-diamine, which upon oxidation in the presence of amines and phenols, yield the dyes of the indamine, indo-phenol, and indaniline classes. The leuco derivatives of these dyes have already been proposed as developers, but their use has been restricted to the