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 fundamental colors, by the admixture of which all other colors are produced. These colors are supposed, by Helmholtz, to be red, green, and violet. All other colors and shades are made from the proper mixture 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 distinct sets of nerves which respond to the wave-lengths of the luminiferous ether which physically represent these colors.
This is all very simple and extremely plausible, but certain phenomena 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 influence 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 intensity.
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 lessened 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 sensations 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 remaining fibers are thus affected. Any color, therefore, which contains, be-
