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496 History of Three-Color Photography
colors and it was only necessary to dye the uncolored spaces with the third color. By this method six or nine colors might be used.
The Vereinigte Kunstseide-Fabriken100 impressed celluloid with fine lines and applied greasy resists to the raised bands, and by immersion in an alcoholic solution of the dye the depressions were dyed. The resist was removed, the depressions were again formed at right angles to the first and the prior operation repeated. After removal of the resist the raised parts were coated with dichromated gelatin, rendered insoluble by light, then dyed up and the sheet flattened by heat and pressure. In an improvement101 the same inventors would utilize the non-absorption of a dye by a previously colored area, during short immersion in a second solution.
The Societe Anonyme des Plaques et Papiers A. Lumiere et ses Fils102 applied a resist to gelatin dyed with one of the colors in a pattern and then destroyed the dye by some chemical agent. A second series of lines was imprinted at right angles to the former and the same operation gone through. The same inventors103 would utilize the principle of basic dyes precipitating acid dyes, as first employed by Powrie. A gelatin-coated support was given a resist as to two-thirds of its surface and the exposed parts dyed orange with eosin scarlet, and the resist removed. A further series of resists were imprinted and the sheet immersed in metanil yellow S, and cyanin V, to which was added some iron salt. This mixture dyed the free parts green, and the iron salt prevented any action on the eosin scarlet. The resist was removed, the remaining parts dyed with methyl violet, and the precipitation of this last dye by the eosin and green dye prevented the penetration of the parts already dyed. As to the use of iron salts see Krayn, page 494.
J. Rheinberg104 proposed to use a colloid as a resist, and sensitized with uranium and iron salts. The colloid used depended on the nature of the underlying film, which acted as a vehicle for the dyes. The governing condition was that both should be permeable to the solvent used for extracting the dye from the underlying film through the upper, or dyeing the lower through the upper. A specific example was : a support was coated with collodion dyed red, then coated with albumen containing 7.5 per cent of ammonio-citrate of iron and an equal quantity of uranium nitrate. This was exposed under a matrix with 200 opaque and 200 clear lines to the inch, the lines being of equal width. The plate was then immersed in acidified alcohol which extracted the red dye from those parts of the collodion film corresponding to the clear lines of the matrix. The plate was then immersed in a green dye, which stained the cleared parts. The plate was washed in water, the resist sensitized as before, and exposed under a matrix with the same number of rulings, but with the opaque lines twice the width of the clear, placed at right angles to the first screen lines, during exposure. After immersion in acidified alcohol to extract the dyes, the violet dye was applied.