COLOR-BLINDNESS AND COLOR-PERCEPTION. 91 



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 as- 

 similation. 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 ac- 

 cording as the dissimilating power of the two remaining colors is in- 

 tense 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 fre- 

 quently 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, dif- 

 ferent 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 shorten- 

 ing of the spectrum, extending, as it sometimes does, into the orange, 

 and even into the yellow. 



4. The line of demarkation 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 vio- 

 let, 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 ad- 

 vanced : 



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 an<J which the D-action. 



