SENSATIONS OF COLOR 



545 



In Fig. 237 the three curves represent schematically, according to Helm- 

 holtz, the relative degree to which each component is stimulated by the 

 different light rays in the production of their appropriate color sensations, 

 thus: 



Simple red stimulates the red-perceiving component strongly, the other two 

 feebly; sensations of red. 



Simple yellow stimulates the red- and green-perceiving components mod- 

 erately, the violet feebly; sensation yellow. 



Simple green stimulates the green-perceiving substance strongly, the other 

 two much more feebly; sensation green. Other effects can be readily combined 

 from the figure. 



Stimulation of all components with about the same intensity gives the 

 sensation of white or of whitish colors. 



According to the three-color theory, black is only an extremely feeble white ; 

 between the two there is no qualitative difference, but only a quantitative one. 



Since according to this scheme the color system of a man with normal 

 vision requires the assumption of three primary colors, the eyes of this class 

 of people are called trichromatic. 



Starting with the Young-Helmholtz theory Konig and Dieterici have car- 

 ried out a very extensive series of measurements and have calculated the form 



FIG. 238. Excitation of the different components of the visual organ by light waves of different 

 wave length, after Konig and Dieterici. 



of their (own) curves of sensation for the three primary colors. They found 

 that at the extreme red and the extreme violet ends of the spectrum they 

 could distinguish a difference of brightness, but no difference of color tone, 

 hence these two portions of the spectrum, up to 655 /*/* and 430 ^ respectively, 

 must stimulate only the red- and the violet-perceiving components. The re- 

 sults which represent graphically the mean value for the two authors are 

 reproduced in Fig. 238. 



But there are eyes with other color systems, eyes for example for which 



