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556 History of Three-Color Photography
M. Obergassner40 suggested to overcome the registration trouble by using coloring substances which were altered by acids or alkalis. For instance, a 25 per cent solution of litmus in gelatin ; a similar strength of metanil yellow, the former being red in acid and blue in alkaline solutions, and the latter violet in acid and yellow in alkalis ; for the green a mixture of acid green, light green Nos. 1 and 2, brilliant yellow and phenolphthalein, which is green in acid but red in the alkaline state. The colors being altered by treatment with acids or alkali. In a later patent41 Obergassner proposed to add some transparent substance to the colloid of the color elements, which should form a lake or additional coloring matter. The substance cited is barium chloride, which is converted into the sulfate, to which dyes adhere. Or silver nitrate might be added and this converted into the iodide, which might be used as a mordant for basic dyes.
O. S. and H. K. Dawson42 would build up the elements on the positive image. From a negative, made under a screen, a positive was obtained and after insulation with varnish, sensitized with a dichromated colloid, and this might be exposed between two matrix screens used for making the negative screen, or the matrices might be successively used to obtain the color elements on the positive, which could be sensitized with a colloid between each printing. The colors were to be obtained by using the selective action of dyes for hard and soft gelatin, for instance, by immersion of a printed colloid in a mixture of safranin, brilliant yellow and naphthol green. The most exposed parts would be colored green and the least exposed red. The third color might be applied in alcoholic solution to the support, which was to be celluloid.
J. H. Christensen43 would produce positives from screen-plates, prepared according to E.P. 25,419, 1913 (see p. 397), with permeable films. Three papers, stained with complementary dyes, were to be brought into contact with a film containing developer, and exposed through the screenplate by light complementary to the color of the dye in the paper. During exposure the developer acted upon the film and the dye diffused into the plate containing the developer.
J. E. Thornton44 would coat paper with a screen pattern, either by a dichromated colloid, or using a silver emulsion, or printing from an intaglio roller or block. Any pattern might be used and the only requisite seems to have been that the mosaic pattern should be larger or smaller than those of the plates to be printed from. No information is given as to how registration was to be done. A subsequent patent45 which is stated to be particularly suitable for cinematographic work, consists in superimposing on celluloid two, three or more absorbent layers, the individual colors being applied to each layer and then the whole coated with emulsion. This might be used for positive and negative work. Any grainy effect might be overcome by slightly altering or staggering the color elements in each