VISION. : 585 
stimulation is not confined solely to the rays of the red end of 
the spectrum, it is chiefly by these that what we may call psy- 
chic red is produced—i.e., the mental perception of red is de- 
pendent on a specific stimulation of the retina by rays of a cer- 
tain wave-length, though at the same time there is a feebler 
sensation of green and violet. Orange would in like manner 
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Fie. 427.—Illustrates the Young-Helmholtz theory of color-vision. The letters in the lower 
line indicate colors of spectrum in natural order. 1, denotes the ‘‘red”; 2, ‘‘green”; 3, 
‘violet’ primary-color sensation. The diagram shows by the height of ‘the curve in 
each instance to what extent the primary-color sensations are respectively excited by 
vibrations of different wave-lengths (after Bernstein). 
3 
result from a large admixture of red, considerable of green, and 
very little of violet. 2. Hering’s theory is a chemical one. He 
assumes the existence of three kinds of visual substances: 
white-black, yellow-blue, red-green. Hither in the retina or 
elsewhere in the eye it is believed that two processes are in con- 
stant operation, the opposite of each other, and which corre- 
spond to the changes assumed to take place in protoplasm 
generally, and to which we have referred already .as ana- 
bolism and katabolism, or construction (assimilation) and de- 
struction (dissimilation). When dissimilation is in excess, the 
lighter colors result—white, yellow, red; and the others when 
assimilation prevails. Orange would be seen when red and 
yellow are simultaneously produced—i. e., when the red-green 
and yellow-blue substances both undergo dissimilation to a 
degree in excess of its opposite phase. 
One test of these theories would be their application to ex- 
plain the defect next to be mentioned. 
Color-Blindness.—There are all degrees of this defect, from 
such as exists in every eye—i.e., inability to perceive color 
equally well by all parts of the retina, to complete loss of the 
faculty of discriminating color at all. 
1. Complete color-blindness (achromatopsy) is marked by 
inability to distinguish any colors, the spectrum being brightest 
