In accordance with this theory, there are, therefore, four fundamental colors instead of three (excluding black and white), namely: red, green, yellow and blue, and they are supposed to be produced as follows: Red is the product of the dissimilation of the red-green substance, green is the result of its assimilation; blue is the result of the dissimilation of the blue-yellow, and yellow of its assimilation. When the A-and D-action on the red-green and blue-yellow substance are equal there is no color sensation, but only the D-action of these colors on the black-white substance, that is white. Simultaneous A-and D-action on the black-white substance, however, is not attended by abolition of sensation, but by the sensation of gray.
It will be seen from this that, in the Hering theory, what were before considered as complementary colors are antagonistic and tend to neutralize each other. It will be remembered that those colors have been called complementary which, when mixed together, would produce white (we speak now of spectral colors). This was explained by the Young-Helmholtz theory on the principle of combination; it is accounted for by the Hering theory on the principle of subtraction. When red and green, for instance, form white on being mixed, the white is not produced by the sum of the sensations of red and green, but the red and green, being antagonistic, neutralize each other, and there only remains the D-action of both colors on the black-white substance—that is, white.
As in the Young-Helmholtz theory, the other colors, aside from the primary, are the results of mixed sensations.
Color-blindness, in accordance with this theory, is of two forms. In one, both color substances are wanting, and there only remains the black-white substance to be acted on by light (achromatopsia). In the other form, one of the two color-substances is lacking and only the two colors of the remaining color-substance are distinguishable (dichromatopsia). If the red-green substance is lacking, there will be red-green blindness or blue-yellow vision; if the blue-yellow substance is the missing one, there will be blue-yellow blindness, or red-green vision.
To satisfactorily account for some of the phenomena of color-blindness, however, it becomes necessary to suppose that, when one color-substance is wanting, the light rays which act specifically on that substance produce an A-or D-action on the remaining color-substance. In red-green blindness, for example, red, yellow, and green act in a dissimilating manner on the remaining blue-yellow substance, giving rise to the sensation of yellow, while blue alone acts in an assimilating manner. The most strongly dissimilating color will be yellow, while the others will be more or less varying in their action. In the case of blue-yellow blindness, red, yellow, and blue are the dissimilating colors and green the assimilating color. It will be readily understood, when we have this state of affairs, that in the dichromatrope, where