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486 History of Three-Color Photography
yellow, such as fast chrome yellow G, and the untanned rectangles of the blue-dyed colloid were stained green. In a subsequent patent55 Faupel proposed to immerse the exposed colloid in a mixture of naphthol green and crystal ponceau. After some time the hardened lines would be stained red and the unhardened green. The film was again sensitized, the dichromate being taken up only by the soft green lines, and then on exposure the uncovered green lines would be hardened. The film was then immersed in a mixture of alcohol and water which decolorized the untanned fields and it was then immersed in brilliant azurin solution, which only took on the colorless parts without affecting those previously dyed.
E. Sanger-Shepherd56 adopted du Hauron's method exactly. A. Lowy" also probably used the dichromate method as a colloid film was used, figures impressed by light and the dye partly retained or partly washed out or bleached. Then a fresh sensitizing which only took on the unhardened parts, fresh exposure and this repeated for the third time. C. Spath58 patented a process which is almost identical with that of Faupel, using ponceau BO and brilliant azurin for the red and blue lines and flavazin for the yellow dye, and the alternative of washing out the dye with alcohol and water and the use of the same dyes is disclosed. The ways of authorities of patent offices are above the understanding of most laymen, and it will be seen that this is not the only instance in which practically or actually identical processes are protected.
J. M. Child59 patented a process of making hexagonal elements by ruling parallel line screens and combining them in three layers with the lines crossing at 60 degrees, to form clear hexagons on a black ground, and printing from the combined film to form a matrix, a special frame60 being used in the dichromated colloid printing. Berthon and Gambs61 printed on dichromated colloid, dyed up with red dye, which was soluble in water, and the plate washed till the dye was removed from the unhardened lines. The plate was then immersed in a blue dye, which only took on the uncolored lines. The plate was then varnished, coated with dichromated colloid dyed yellow. Exposure was made with the matrix lines at 45 degrees to the first lines, and after washing was dyed up in blue. The plate thus showed blue lines with yellow interspaces, but by superposition with the first screen they were crossed by the red and blue and thus gave green and violet.
J. M. Borrel62 utilized this common idea, and the plate was stained in red, which took on the exposed parts, the chromium salt being said to act as mordant. It was then stained blue which took only on the unexposed and after again sensitizing and printing with the matrix lines at right angles to the first it was stained yellow. The result was orange lines with green and blue rectangles, and immersion in a red dye, which acted only on the blue, formed violet. R. Goldschmidt63 and F. M. Duncan64 also used the dichromate method. B. Bichtler65 coated both sides of a support with