THE SENSE OF SIGHT. 



633 



green, blue, and violet the so-called spectral colors, so well exem- 

 plified in the rainbow. The spectral colors are termed simple 

 tions of the ether, from about 400 millions of millions per second for 

 Objectively, the spectral colors consist of very rapid transverse vibra- 

 colors, because they can not be any further decomposed by a prism, 

 red to about 760 millions of millions for violet, but subjectively they 

 are sensations caused by the impact of the ether- waves on the per- 

 cipient layer of the retina. 



It is possible to mix or blend these spectral color-sensations in 

 the eye by stimulating the same area of the retina by different spectral 

 colors, either at the same time or in rapid succession. The following 

 table shows the results of such experiments as performed by v. Helm- 

 holtz (Dk. = dark; Wh. == whitish): 



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

 new color-sensations can be produced, white and purple, the remain- 

 ing 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 pigments. 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 de- 

 pending 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 



