246 THE HUMAN BODY 



and the fact that pairs or groups of fused colors give rise to sen- 

 sations entirely unrelated to any of the constituent colors. The 

 theory must account for the distribution of color perception over 

 the retina and for the facts of color blindness ; it must also explain 

 after-images and contrasts. The fact that black, the absence of 

 stimulation, has all the subjective qualities of a true sensation is 

 also to be explained in some way. No theory yet proposed is 

 satisfactory in accounting for all the known facts. Each one 

 lays special emphasis on some group of visual phenomena and 

 disregards such facts as cannot be harmonized with it. Three 

 interesting theories will be briefly summarized for the sake of 

 showing how such a problem is attacked. Each of them assumes 

 that the excitation of the visual nerve endings depends upon the 

 action of light upon certain photochemical substances in the cones. 



The Young-Helmholtz Theory. This theory, proposed by 

 Young in 1807 and elaborated by Helmholtz many years later, 

 may be described rather as an attempt to apply the doctrine of 

 specific nerve energies to color vision than as an attempt to ex- 

 plain the facts of color vision as we know them. 



It is an interesting illustration of the extent to which this doc- 

 trine has come to physiologists to seem fundamental in forming 

 conceptions of the nervous system that the theory of Young, 

 manifestly impossible as it is, because of the numerous facts with 

 which it cannot be harmonized, has received much more atten- 

 tion and consideration than other theories, agreeing with many of 

 the facts as we know them, but not in accord with the doctrine 

 of specific nerve energies. 



The theory assumes all our color sensations to be based on three 

 primary ones, red, green, and violet, each of which is aroused by 

 the decomposition of its special photochemical substance, and 

 each having distinct nervous connection with the visual area of 

 the cerebrum. Since anatomical study shows that each cone has 

 a single nerve-fiber leading from it we must either suppose that 

 there are three sorts of cones, one red-perceiving, one green- 

 perceiving, and one violet-perceiving, and that these are scattered 

 in groups of three over the retina; or we must conclude that the 

 nerve-fiber is not the unit of nervous conduction but that it is 

 made up of smaller units in the same way that the nerve-trunk is 

 made up of fibers. The originators of the theory held the first 



