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276 History of Three-Colo?' Photography
and 40 per cent alcohol, and stated "by this means the red dye, which is more soluble in water, is carried into the film, while the blue dye, which is more soluble in alcohol, remains substantially on the surface of the plate." In a later patent138 Selle claimed the use of dyes, which are insoluble or not readily soluble in water, but dissolved by the aid of alkali or acid, then immersing the plate in baths of the opposite character so as to regenerate the dye. As an example is given the use of cyanin in acidulated solution, and the after treatment of the bathed plate in an alkaline solution.
A. Miethe and A. Traube139 patented the use of ethyl red. The Farbenfabrik vorm. Bayer140 patented the use of the dye formed by the action of diethyl sulfate on quinaldin. This being also patented by E. Berendes.141 Later the use of tolu-quinaldin, the homologue of quinaldin was claimed, and this yielded isocol.142 Also the use of additional products of quinaldin and the corresponding quinolin derivatives which gave homocol.143 The Compagnie Paris Couleur d'Aniline144 patented the formation of basic red and violet dyes from dinitro-pyridin or cyan-pyridin, but apparently these were not introduced as sensitizers. The Aktien-Gesellschaft f. AnilinFabrikation patented145 dyes form a-naphthol-alkyl-quinaldins. Also the production of dyes by the action of oxidizing agents, such as ferricyanides, persulfates, etc., on quinolin, quinaldin and similar bases.146.
H. Schmidt147 pointed out that it had not been possible to obtain real red sensitiveness even with ethyl violet and that a mixture of dyes, such as erythrosin and cyanin did not give good results, as the former acted best in alkaline and the latter in acid baths, followed by alkaline baths. He proposed to use first an acid cyanin bath and then an alkaline erythrosin bath. F. Tarsulat148 claimed the use of di-iodo-fluorescein either alone or with other dyes, for sensitizing collodion emulsion.
Farbwerke vorm. Meister, Lucius and Bruning149 patented the various isocyanin dyes150 151> 152> 153> 154> 155> 156.
F. F. Renwick and A. Bloch157 patented the use of auramin with the isocyanins and claimed increased sensitiveness for red and green, together with cleaner plates and better keeping properties. For bathing the mixture might be used in 1 : 50,000 with an equal quantity of isocyanin ; time of bathing 3 to 4 minutes. If the dyes were to be mixed with the emulsion from 0.005 to 0.025 g. of each dye should be added to 1 liter. If a selfscreened plate were desired then 1 g. : 50,000 should be used for bathing and 0.25 g. added to 1 liter of emulsion.
The author158 called attention to the following note by J. W. Gifford :159 "I wish to make the following communication that photographic gelatin films, bathed in a solution of auramin and erythrosin, the latter in small proportion, give an approximation to correct color values without the use of any screen or light filter, and are sensitive to all colors, including red. N.B. there are other dyes than auramin which besides sensitizing the silver salt stain the gelatin film, which then plays the role of a light