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Historical and Theoretical Data 29
the result when all three colors are mixed together is not black, but brown. In consequence of the ignorance of this fact, it is contended that three-color printing can not produce black and grey, and thence has been deduced the necessity of a neutral grey plate: When using blue-green, crimson and yellow, there is nothing, from a theoretical point of view, that stands in the way of the formation of black or grey.
The third reason why blue-green, crimson and yellow must be chosen as fundamental colors lies in the difficulty or impossibility of splitting up the colors of the original by photographic means into other colors. The negative for the yellow printing should have the blue and red opaque ; the yellow, on the other hand, should be clear glass. This requirement is only fulfilled when crimson is chosen as the fundamental color for red, which must reflect the whole of the blue rays, and therefore act on the blue-sensitive plate as white. A pure yellow can never be photographed with red so that the former acts like white and the latter like black, for the red color reflects almost exclusively the orange and red rays ; the yellow pigment, on the other hand, reflects the green, yellow, orange and red rays.
We must have a plate which will reproduce the red like white, the yellow like black. Even if one sensitizes for the extreme red, the yellow pigment will still act, for it reflects red rays. This fact can not be gotten over by the most vigorous sensitizer nor by any filter. It is only possible to split up the original colors into three fundamental colors, which show as far as possible striking differences, for only then can the requirements, that each of the three negatives should have two colors opaque and the third transparent, be fulfilled. With the fundamental colors — blue-green, crimson and yellow — this can be easily done. A crimson pigment of the particular hue of dry erythrosin collodion reflects red, violet and blue rays. A yellow dye, however, as already mentioned, reflects green, yellow, orange and red rays. A violet-blue sensitive plate will, therefore reproduce the former like white, the yellow like black, and thus separate the two colors in the desired way. The vermilion-red of the original will then be formed by a mixture of equal parts of yellow and crimson, for it will be quite satisfactory if the vermilion be equally inactive for the yellow and red printing. If, however, one uses for printing a carmin lake then there will always be formed an orange instead of the vermilion.
It will thus be seen how important it is to choose the correct fundamental inks, and that it can not be dismissed with the apodictic statement that "only three stable inks are required." If there are at present no stable suitable inks we must seek for them, and if we can not obtain them we must be content to use the best available and take into account the consequent faults. If pure greens are wanting, if orange replaces vermilion and violet-blue, if we can obtain no black or greys, if the negatives, in spite of correct filters, will not show the correct appearance, we must seek