PROPERTIES OF THE RETINA. 351 



being a system of lenses or mirrors by which different portions of 

 a spectrum can be superposed. The usual device employed in 

 laboratory experiments is that of rotation of discs of colored paper. 

 Each disc has a slit in it from center to periphery so that two cliscs 

 can be fitted together to expose more or less of each color. If a 

 combination of this kind is attached to a small electrical motor it 

 can be rotated so rapidly that the impressions of the two colors 

 upon the retina follow at such a short interval of time as to be prac- 

 tically simultaneous. 



The Fundamental Colors. By the methods of color fusion 

 it can be shown that three colors may be selected from the spec- 

 trum whose combinations in different proportions will give white, 

 or any of the intermediate color shades, or purple Considered 

 purely objectively, a set of three such colors may be designated 

 as the fundamental colors, and red, yellow, and blue, or red, 

 green, and violet have been the three colors selected. On the 

 physiological side, however, it has been assumed that there are 

 certain more or less independent color processes photochemical 

 processes in the retina which give us our fundamental color sen- 

 sations, and that all other color sensations are combinations of these 

 processes in varying proportions with each other or with the proc- 

 esses causing white and black. Referring only to the colors proper, 

 the fundamental color sensations according to some views are red, 

 green, and blue or violet; according to others, they are red, yellow, 

 green, and blue. (See paragraph on Theories of Color Vision.) 



Helmholtz calls attention to the fact that the names used for these funda- 

 mental color sensations are obviously of ancient origin, thus indicating that 

 the difference in quality of the sensations has been long recognized. Red is 

 from the Sanskrit rudhira, blood; blue from the same root as blow, and re- 

 fers to the color of the air; green from the same root as grow, referring to the 

 color of vegetation. Yellow seems to be derived from the same root as gold, 

 which typified the color. The other less distinct qualities have names of 

 recent application, such as orange, violet, indigo blue, etc. 



Complementary Colors. It has been found by the methods of 

 color fusion that certain pairs of colors when combined give a white 

 (gray) sensation. It may be said, in fact, that for any given color 

 there exists a complement such that the fusion of the two in suitable 

 proportions gives white. If we confine ourselves to the spectral 

 colors we recognize such complementary pairs as the following: 



Red and greenish blue. 



Orange and cyan blue. 



Yellow and indigo blue. 



Greenish yellow and violet. 



The complementary color for green is the extraspectral purple. 

 Colors that are closer together in the spectral series than the 



