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Historical and Theoretical Data 15
not, therefore, equally apply to those material colors, which we have to reproduce by photography. If such primary colors are used in optical synthesis, only very whitish compound colors will be formed, and in subtractive synthesis very black compound colors. From these considerations it ought to be clear that the Young-Helmholtz theory of color vision possesses no importance for the theory of three-color photography, and that it is not correct, therefore, to identify the physiological fundamental colors with those of trichromatic photography.
As a further proof that the fundamental color-sensation curves have nothing to do with three-color work, it may be as well to mention that some have enunciated the law that each of the three taking plates should excite one of the nerve fibrils, and the niters must be so chosen that they transmit the rays of the spectrum in the intensity ratios of the primary sensation curves. If, for instance, Konig's or Abney's curves are adopted, it will be seen that the red curve is composed principally of yellow, orange and yellowish-green light with less red and green but with some blueviolet. The color of a filter, that would transmit such a curve, would be a brownish-yellow, which can be made by mixing solutions of acridin yellow and naphthol green. But it is obviously absurd to use this for three-color work, even with a perfect panchromatic plate it would give useless results. Exactly in the same way it will be found that the filter corresponding to the primary green sensation curve must transmit chiefly the green, but also some orange, red and pure blue, and it would be a brownish-green. These observations again show that the Young-Helmholtz theory can not be satisfactorily employed in three-color work.
Von Hiibl26 further pointed out that the obvious connection between the filters and the printing inks has been frequently stated, and that Cros and du Hauron started with this. A green or blue or yellow filter was used and the negatives thus obtained printed in red, yellow and blue. And the same principle led K. Hazura and O. Hruza27 to carry out their researches on the subject.
The expression "complementary" is a little indefinite, as it is possible with a given red to find several greens that are complementary by means of the color top, yet the greens may have totally different spectral characteristics. For instance, it is possible to find a green that absorbs the red and another that reflects only the green of the spectrum. Both are actually of the same color and complementary to red, but if used as filters would give totally different results. The shade plays no part even with complementary colors ; one given green is not only complementary to a given red but also to its mixtures with black, thus to brown, and the red would not only give with the pure green, but also with olive, a colorless color-top mixture. The requirement that the filter shall be complementary to the printing ink may perhaps be satisfactory once, but another time it may be false. It is quite another thing, however, if to the