COLOR PERCEPTIONS. 587 



called the spectrum. The white solar light is thus shown to be a 

 .compound of rays of several colors which possess different de- 

 grees of refrangibility, and hence are separated on their way 

 through the prism. The violet rays are the most bent, and the 

 red the least, so that these form the two extremes of the visible 

 spectrum. The difference of color depends upon the different 

 lengths of the waves, the vibrations of violet (667 billions per 

 sec.) being much more rapid than those of red (456 billions per 

 sec.). Beyond the visible spectrum at the red end there are other 

 rays which, though they look black to the eye, are capable of 

 transmitting heat. This thermic power is best developed in these 

 ultra-red rays, and fades gradually towards the middle of the 

 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. 



