86o COLOR-BLINDNESS: ITS PRACTICAL IMPORTANCE. 



process of metabolic change; while during rest it must be renewed, or undergo 

 assimilation. For the perception of white (light) and black (dark) Hering assumes 

 two different chemical processes in the visual substance, namely that the sensation 

 of white or light corresponds with dissimilation (decomposition) , that of black 

 (dark) with assimilation (reconstruction) of the visual substance. Accordingly, 

 the different degrees of distinctness or intensity with which these two sensations 

 appear in the various transitional shades from pure white to deepest black, or, 

 in other words, the proportions in which they appear to be mixed (gray) , corre- 

 spond to the relative intensity of these two psychophysical processes. Conse- 

 quently the consumption and the restoration of the visual substance are the 

 primary processes in the perception of white and black. The consumption of 

 the visual substance in the perception of white is the result of the stimulation 

 of the vibrating ether- waves, and the degree of the perception of brightness is 

 proportional to the amount of material consumed. The restoration of the material 

 produces the sensation of black; the more intensely this takes place the deeper 

 is the sensation. The consumption of the visual substance in one place evokes 

 greater reproduction in the neighborhood. Each process influences the other 

 simultaneously and conjointly. In this way a physiological explanation is pro- 

 vided for the phenomenon of contrast (see p. 864), for which the older view could 

 offer only a psychical interpretation. 



In an entirely analogous manner, color-sensation is regarded as a sensation 

 of decomposition (dissimilation) and of reconstruction (assimilation) . In addition 

 to white, red and yellow are the expression of decomposition; green and blue, 

 on the other hand, represent the sensation of reconstruction. The visual substance 

 is, thus, subject to three different forms of chemical change, or metabolism. 

 The colored contrast-phenomena or after-images are thus explained. The black- 

 white sensation may, further, be combined with all of the colors. It gives a dark 

 or a light tone to each color-sensation, so that there are no absolutely pure colors. 

 There are, thus, three different constituents of the visual substance: that which 

 is sensitive to black-white (colorless), that sensitive to blue-yellow, and that 

 sensitive to red-green. All rays of the visible spectrum decompose the black- 

 white substance, but the different rays do so in different degrees. Only certain 

 rays, on the contrary, decompose the blue-yellow, or the red-green substance; 

 others reproduce them; and still others have no effect whatever. Mixed light 

 appears colorless when it causes an equally strong dissimilation and assimilation 

 in the blue-yellow and the red-green substances, so that the two processes neutralize 

 each other, and the action upon the black- white substance alone appears. Two 

 objective kinds of light, which together yield white, are consequently not to be 

 considered as complementary, but as antagonistic, for they do not combine to 

 form white, but, as antagonists, allow it to appear of itself because each neutralizes 

 the effect of the other. 



The weakness of the Young-Helmholtz color-theory lies in the fact that it 

 assumes the existence of only one kind of irritability, stimulation and exhaustion 

 (corresponding to Hering's dissimilation), and that it ignores the antagonistic 

 relations of certain light-rays to the eye. Therefore, it does not recognize that 

 white is produced from complementary colors by the neutralization of their 

 action in the colored visual substances, but by their supplementing each other. 

 , . . * n . Applying Hering's theory to color-blindness, it must be assumed that the red- 

 blind individual has no red-green visual substance. His solar spectrum con- 

 tains only two partial spectra: the black- white and the yellow-blue. The green 

 part appears colorless to him; the rays from the red portion are visible to the 

 extent that the yellow and the white sensations that they arouse are strong enough 

 to stimulate the retina .sufficiently. He divides his spectrum into a yellow 

 and a blue half. The violet-blind individual has no yellow-blue visual substance. 

 His spectrum contains only two partial spectra: the black- white and the red- 

 green. In cases of complete color-blindness both the yellow-blue and the red- 

 green visual substances are absent, and the individual has only the sensation of 

 Lgnt and dark. The sensibility to light and the length of the spectrum are 

 preserved; the brightest area is in the yellow, just as in the normal eye. 



COLOR-BLINDNESS: ITS PRACTICAL IMPORTANCE. 



By color-blindness (dyschromatopsid) is understood a pathological condition. 



as the result of which the affected individuals are unable to recognize certain 



t was recognized by Tuberville in 1684, and by Huddart in 1777, but 



