THE SENSE OF SIGHT 



725 



violet the so-called spectral colors, so well exemplified in the rainbow. 

 The spectral colors are termed simple colors, because they cannot be any 

 further decomposed by a prism. Objectively, the spectral colors consist of 

 very rapid transverse electro-magnetic vibrations of the ether, from about 

 400 millions of millions per second for red to about 760 millions of millions 

 for violet, but subjectively they are sensations caused by the impact of the 

 ether-waves on the percipient 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. Helmholtz (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 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 30 



supe 

 [nfer 



Inferiorly 70 



60 



o 

 35 



