OPTICS 275 



appear whiter, less saturated, than the corresponding spec- 

 tral colors. For example, spectral red and yellow mixed 

 form an orange color which appears whiter than the orange 

 of the spectrum. 



Complementary colors are two colors which by mixing 

 give the sensation of white. The following are pairs of 

 complementary colors: red and greenish blue, orange and 

 blue, green and indigo blue, greenish yellow and violet. 



The theories of color reduce the many color sensations to the 

 simultaneous but unequal stimulation of a few primary colors. The 

 Young-Helmholtz theory assumes three primary color sensations 

 red, green, and blue, which by the action of a light are always 

 stimulated simultaneously but unequally by the individual homo- 

 geneous rays. By the mixing of the various primary colors, the 

 different color sensations are produced. If the three color sensa- 

 tions are equally stimulated, the sensation of white is produced. 



Recently the idea has been advanced that only the cones of the 

 retina are the apparatus for the color sensations, and that with 

 these are connected three classes of nerves corresponding to the 

 three primary color sensations. The rods are supposed to be sen- 

 sitive only to white light of low intensity, and this sensation is 

 brought about by the decomposition of the visual purple. Accord- 

 ing to this view, only the rods are stimulated by light of very low 

 intensity, which therefore appears colorless, while such light is not 

 able to stimulate the cones. In dim light we therefore see chiefly 

 with the rods, in bright light with the cones. 



Bering's theory of opposite colors assumes six primary color- 

 sensations, which are classified into three groups of two sensations 

 each: 



i. White and black ; 2. Red and green ; 3. Blue and yellow. 

 For each group a visual substance is assumed by whose changes 

 sensations are produced. The changes of the substance may be of 

 two kinds ; the one kind produces one sensation, the other, oppo- 

 site in character to the first, produces the other sensation. Of the 

 two opposite processes, one is supposed to be the dissimilation, the 

 other the assimilation, of the visual substance. 



The retina is sensitive to colors only in its central portion. 

 As we proceed to the periphery the power of discerning 

 colors decreases. The outermost parts of the retina are 

 color-blind. 



Besides the color-blindness in the peripheral region of the retina, 

 pathological color-blindness of the whole retina occurs. 

 This color-blindness may be : 



