PROPERTIES OF THE RETINA. 361 



have been submitted to satisfactory experimental verification. The 

 immediate stimuli that give rise to the visual impulses are assumed 

 to be of a chemical nature, and it seems probable that in this 

 case as in that of many other problems of physiology, we must 

 await the development of a more complete knowledge of the 

 chemical processes involved. The theories proposed at present, 

 while all tested by experimental inquiries, are in a large measure 

 hypotheses constructed to fit more or less completely the facts that 

 are known. Three of these theories may be described briefly as 

 examples of the modes of reasoning employed: 



I. The Young-Helmholtz Theory. This theory, proposed essen- 

 tially by Thomas Young (1807) and afterward modified and ex- 

 panded by Helmholtz,* rests upon the assumption that there are 

 three fundamental color sensations, red, green, and violet and 

 corresponding with these there are three photochemical substances 

 in the retina. By the decomposition of each of these substances cor- 

 responding nerve fibers are stimulated and impulses are conducted 

 to a special system of nerve cells in the visual center of the cerebrum. 

 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 

 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. 153) 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 



* Helmholtz, " Handbuch der physiologischen Optik" second edition. 

 1896, I, 344. 



