332 



THE SPECIAL SENSES. 



The theory, therefore, assumes special nerve fibers and nerve centers 

 corresponding respectively to the red, green, and violet photo- 

 chemical substances, and the peculiar quality of the resulting sensa- 

 tions are referred, in the original theory, to the different reactions 

 in consciousness in the three corresponding centers in the brain. 

 When these three substances are equally excited a sensation of 

 white results, of greater or less intensity according to the extent of 

 the excitation. White, therefore, on this theory, is a compound 

 sensation produced by the combination or fusion in consciousness 

 of the three equal fundamental color sensations. The sensation of 

 black, on the other hand, results from the absence of stimulation, 

 from the condition of rest in the retina and in the corresponding 

 nerve fibers and nerve centers. All other color sensations yellow, 

 for instance are compound sensations produced by the combined 



R 



B 



Fig. 144. Schema to illustrate the Young-Helmholtz theory of color vision. (Helm- 

 holtz.) The spectral colors are arranged in their natural order, red to violet. The curves 

 represent the intensity of stimulation of the three color substances: 1, The red perceiving 

 substance; 2, the green perceiving; 3, the violet perceiving. Verticals drawn at any 

 point of the spectrum indicate the relative amount of stimulation of the three substances 

 for that wave length of the spectrum. 



stimulation of the three photochemical substances in different propor- 

 tions. It is assumed, furthermore, that each of the photochemical 

 substances is acted upon more or less by all of the visible rays of the 

 spectrum, but that the rays of long wave lengths at the red end 

 of the spectrum affect chiefly the red substance, those corresponding 

 to the green of the spectrum chiefly the green substance, and the 

 rays of shortest wave length chiefly the violet substance. These rela- 

 tionships are expressed in the diagram given in Fig. 144. The figure 

 also indicates that it is impossible to stimulate any one of these sub- 

 stances entirely alone, that is, we cannot obtain a perfectly satu- 

 rated color sensation. Even the extreme red or the extreme violet 

 rays act more or less on all of the substances, and the resulting red 

 or violet sensation, is, therefore, mixed to some extent with white, 

 that is, is not entirely saturated. The theory, as stated by Helm- 

 holtz, held strictly to the doctrine of specific nerve energy, in assuming 



