COLOR PERCEPTIONS. 589 



spectrum. Outside the violet are ultra-violet rays, which, though 

 non-exciting to the retina, are very active in inducing many 

 chemical changes. Only those other vibrations which have a 

 medium length can stimulate the retina. 



If two different colors be mixed before reaching the retina, or 

 be applied to it in very rapid succession one after the other, an 

 impression is produced which differs from both the colors when 

 looked at separately ; thus, violet and red give the impression of 

 purple, a color not in the spectrum. If all the colors of the spec- 

 trum in the same proportion and with the same brightness fall 

 upon the retina, the result is white light. This we know from 

 the common experience of ordinary white light, which is really 

 a mixture of all the colors of the spectrum, and we can see it 

 with a " color top " painted to imitate the colors of the spectrum. 

 When the top is spinning, the colors meet the eye in such rapid 

 succession that the stimulus of each falls on the retina before that 

 of the others has faded away, and thus many colors are practi- 

 cally applied to the retina at the same time, and the top looks 

 nearly white. 



It has been found that certain pairs of colors taken from the 

 spectrum when mixed in a certain proportion produce white. 

 These are complementary to one another. The complementary 

 colors are : 



Red and peacock-blue, Yellow and indigo, 

 Orange and deep-blue, Greenish-yellow and violet. 



If colors which lie nearer to each other in the spectrum than 

 these complementary colors be mixed, the result is some color 

 which is to be found in the spectrum between the two mixed. 



The perception of the vast variety of shades of color that we 

 can distinguish can only be explained by means of this color 

 mixing. We assume that there are three primary colors which 

 overlap one another in the spectrum so as to produce all the 

 various tints. These are red, green and violet ; the arrangement 

 of which may be thus diagrammatically explained (Fig. 232). 



We must further assume that there are in the retina three 

 special sets of nerve terminals, each of which can only be stimu- 

 lated by red, green or violet respectively, and the innumerable 



