COLOR-BLINDNESS AND COLOR-PERCEPTION 87 



Such, however, is no longer the case, and there are many who are 

 not almost but quite persuaded that the true theory of vision is one 

 of the questions to be solved by the coming physiologist. This theory 

 of Young-Helmholtz, as it is called, demands three primary or funda- 

 mental colors, by the admixture of which all other colors are pro- 

 duced. These colors are supposed, by Helmholtz, to be red, green, 

 and violet. All other colors and shades are made from the proper mixt- 

 ure of two or more of these colors. White is the sensation produced 

 by the proper mingling of all three sensations ; black is the absence 

 of sensation. Corresponding to these three primary sensations there 

 are in the retina, or terminal expansion of the optic nerve, three dis- 

 tinct sets of nerves which respond to the wave-lengths of the luminif- 

 erous ether which physically represent these colors. 



This is all very simple and extremely plausible, but certain phe- 

 nomena of vision make it necessary to so modify this simplicity as to 

 spoil its beauty and give an elasticity to the theory which can not be 

 gratifying to the student of exact science. It becomes necessary to 

 suppose, for instance, that the nerve-fiber which responds to red is also 

 affected, in a less degree, by the green waves, and in a still less degree 

 by the violet; and the green waves, while principally affecting the 

 green fibers, affect also the red and violet; and the violet waves influ- 

 ence the red and green fibers, though in a much less degree than they 

 do the violet. In this theory gray is but a white of diminished in- 

 tensity. 



Color-blindness is explained in keeping with this theory as follows : 

 Any one or all three of the color-fibers may be wanting, or lacking in 

 functional activity. Consequently there may be red-, green-, or violet- 

 blindness, or there may be total color-blindness. Since, however, it is 

 supposed that each one of the color-fibers is affected (though in a less 

 degree) by both other colors as well as by its own peculiar color, there 

 must be a sensation produced by each color, though it will be of less- 

 ened intensity in the case of the lacking color, and that sensation must 

 be other than that of the color belonging to the missing fiber. Under 

 these circumstances, even a saturated primary color would not, when 

 its fiber was missing, appear black, though it would appear darker than 

 to one with normal color-perception. To a red-blind person, a spectral 

 red, for example, while appearing a color much less luminous than is 

 usual, would not be black ; and, if a solar spectrum were presented to 

 such a color-blind individual, it need not appear shortened at the red 

 end. If the green fiber is the lacking one, green will not appear as 

 black,' but when of a certain shade will appear as gray, and for the 

 following reason : White is the product of the sum of all the sensa- 

 tions which the mind is capable of perceiving through the eye. When 

 the eye is normal, we have it when all three of the fibers are affected 

 in about the same degree, and in the color-blind when the two remain- 

 ing fibers are thus^affected. Any color, therefore, which contains, be- 



