674 TEXT-BOOK OF PHYSIOLOGY. 



These are the mixed colors. But it is to be observed that only two new color- 

 sensations can be produced, white and purple, the remaining mixed colors 

 already finding their equivalent in the spectrum. White and purple, 

 therefore, are color-sensations which have no objective equivalent in a 

 simple number of ether- vibrations like the spectral colors. 



Two spectral colors which by their mixture produce the sensation of 

 white are called complementary colors. Such are red and green-blue, golden 

 yellow and blue, green and violet. The mixture of all the spectral colors 

 produces white again. This is the result of adding two or more color- 

 sensations. Different results are obtained, however, by adding color pig- 

 ments. Yellow and blue, for example, produce in the eye white, but on the 

 painter's palette green. The colors of nature are usually mixtures of simple 

 colors, as can be shown by spectroscopic analysis or by a synthesis of spectral 

 colors. 



In all color-sensations we must distinguish three primary qualities: (i) 

 hue; (2) purity or tint; (3) brightness or luminosity. The first quality gives 

 the main name to the color e.g., red or blue this depending on the spectral 

 color or the mixture of two spectral colors with which it can be matched. 

 The second quality, the tint, depends on the admixture of white with the 

 ground color; and the third quality, brightness, depends on the objective 

 intensity of the light and the subjective sensitiveness of the retina. Color- 

 perception thus far refers only to the most sensitive part of the retina. At 

 the more peripheral parts of the retina the colors are seen somewhat differ- 

 ently, as is shown by the following table giving the limits up to which the 

 colors are recognized: 



White. Blue. Red. Green. 



Externally 90 80 65 50 



Internally . 60 55 50 40 



Superiorly 45 40 35 3 



Inferiorly 70 60 45 35 



Theories of Color-perception. The theory oj v. Helmholtz, originated 

 by Thomas Young (1807), assumes in its latest form the existence in the 

 human retina of three different kinds of end-organs, each of which is loaded 

 with its own photo-chemical substance capable of being decomposed by a 

 certain color, and thus exciting the fiber of the optic nerve. 



In the first group these end-organs are loaded with a red-sensitive sub- 

 stance, which is affected mainly by the red part of the spectrum; the second 

 group has its end-organs provided with a green-sensitive substance, which 

 is mainly excited by the green color; while the third group is provided with 

 a blue-sensitive substance, this latter being mainly affected and decomposed 

 by the blue- violet portion of the spectrum. All these three different end- 

 organs are present in every part of the most sensitive area of the retina, and 

 are connected by separate nerve-fibers with special parts of the brain, in the 

 cells of which each calls up its separate sensation of red or green or blue. 



Out of these three primary color-sensations all other color-sensations 

 arise. If a light mainly excites the red- or green- or blue-sensitive substance 

 of a retinal area, we term it red, green, or blue, respectively. But if two of 

 these photo-chemical substances are stimulated simultaneously, quite differ- 

 ent sensations arise. Thus simultaneous stimulation of the red and green 



