the A-and D-action of the one remaining color-substance are equal, gray will be the result, because, as we have before remarked, where two colors neutralize each other there still remains the action of both on the black-white substance, which will give rise to the sensation of gray or white of diminished intensity. But the same colors will not appear gray to all color-blind persons, for the reason that the same colors do not act in every case with the same intensity of dissimilation and assimilation. In most individuals it is the purple and the blue-green which give rise to the impression of gray.
A spectrum should, in accordance with this theory, appear in only two colors to the color-blind, and may or may not be shortened according as the dissimilating power of the two remaining colors is intense or very feeble. The only colors, of course, which such a color-blind person can with certainty distinguish are the two belonging to the one remaining color-substance, blue and yellow, for instance, when there is red-green blindness, and red and green when there is blue-yellow blindness. It is not to be understood, however, that such an individual can never correctly distinguish other colors. Most frequently he can, but there is always a liability to confusion, often of the most astonishing character; and, moreover, the distinctions are made, not by the sense of color, but by some other characteristic, different degrees of luminosity, most commonly.
The evidences which the phenomena of color-blindness have brought against the three-fiber theory of Young-Helmholtz are:
1. That the red-blind can not distinguish perfectly the greens and violets, nor the green-blind the reds and violets; yellow and blue being the only colors about which they make no mistakes.
2. Even in a spectrum which is very much shortened the red-blind finds the brightest place, not in the bluish-green, as we should expect, but in the yellow, as in the normal eye.
3. This theory can not satisfactorily explain the extreme shortening of the spectrum, extending, as it sometimes does, into the orange, and even into the yellow.
4. The line of demarcation in the spectrum is sharply at the blue, all to the left almost always appearing of one color, and all to the right of another, there being no lines of division between blue and violet, nor between the red and yellow and the yellow and green.
5. The gray or neutral band is far from being invariably present, and when it is it is often, in the red-blind, in the position it should be in the green-blind, and vice versa (Mauthner).
Against the Hering theory the following objections have been advanced:
1. There is no reason for supposing that red and green and blue and yellow are opposing colors. They are all active in their specific line, and even Hering has not been able to determine which possesses the A-action and which the D-action.