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28 History of Three-Color Photography
The theoretic inks are unstable anilin colors, and they have but poor covering power and reflect mostly too much white, that is, they will not give good blacks, and variations in the flow produce great variations in the pulls. For the permanent inks the yellow should be of a slightly greener hue than is generally employed, because the red as a rule contains too much violet. Practically if the position of the red and blue be moved then the yellow must be shifted also. With regard to the permanent inks, both the red and blue lack luminosity and are usually deficient in violet for the red, whilst the blues are deficient in green. Variations in the yellow are more noticeable than in the other colors. The three inks must be of fairly equal working consistency, and when superposed should produce a neutral grey or black. The yellow and red superposed in equal proportions should produce a scarlet, approximating the primary red sensation. The yellow and blue in the same way should give an emerald green approaching the F line, and the red and blue should make a violet midway between ultramarine and methyl violet.
Von Hubl43 naturally dealt with this subject at some length in his book, but probably the best summary of his views is as follows : The three colors must so behave that mixtures of any two must be of equal purity. These requirements are only satisfied by the colors, yellow, crimson and blue-green, for these give an equally pure vermilion-red, violet-blue and green. The colors chrome yellow, carmin lake and Paris blue give only an impure green, the blackness of which can only be estimated by comparison with a pure green. A blue has been recommended as a printing ink, which approximates to ultramarine, because it was assumed that pure blue, red and yellow must correspond best to theoretical requirements. Pure blue, that is one which excites the blue sensation without the secondary excitation of green or red, is, however, complementary to yellow, and can, therefore, under no conditions be used to form a useful green. The green present in a painting is never pure, but consists of green and black, and photography decomposes it into its three constituents — blue, yellow and red. Only when the blue and yellow give a pure green will the necessary degree of sadness (or blue blackness) be formed; if, however, the mixture of blue and yellow is already degraded, there will be formed by the subsequent addition of the red only a greenish-grey. For the existing three-color prints the deficiency in the green is characteristic; it is solely to be ascribed to the use of incorrectly chosen fundamental colors, and only to be avoided by the use of blue-green, crimson and yellow as printing inks.
But there are two further reasons in support of the choice of the above-mentioned fundamental colors. The intensity of tlie three inks must obviously be so chosen that red and yellow, yellow and blue, and blue and red produce the mean sensations orange, green and violet. If we choose, however, such intensities in Paris blue, carmin lake and yellow,