322 THE SPECIAL SENSES. 



or more color processes in the retina, this end being obtained by 

 superposing upon the same portion of the retina the rays giving 

 rise to these color processes. It must be borne in mind that color 

 fusion upon the retina is quite a different thing from color mixture 

 as practised by the artist. A blue pigment, such as Prussian blue, 

 for instance, owes its blue color to the fact that when sunlight falls 

 upon it the red-yellow rays are absorbed and only the blue, with 

 some of the green, rays are reflected to the eye. So a yellow pig- 

 ment, chrome yellow, absorbs the blue, violet, and red rays and 

 reflects to the eye only the yellow with some of the green rays. A 

 mixture of the two upon the palette will absorb all the rays except 

 the green and will therefore appear green to the eye. If, however, 

 by means of a suitable device, we throw simultaneously upon the 

 retina a blue and a yellow light, the result of the retinal fusion is 

 a sensation of white. Many different methods have been employed 

 to throw colors simultaneously upon the retina, the most perfect 

 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 discs 

 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 and the Complementary Colors. By the 

 methods of color fusion it can be shown that three colors may be 

 selected from the spectrum whose combinations in different propor- 

 tions 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- 



