278 THE HUMAN BODY 



may speak of these as red, yellow, blue, etc., rays; ail together, 

 in about equal proportions, they arouse the sensation of white. 



Peculiarities of Color Vision. A remarkable fact is that most 

 color feelings can be aroused in several ways. White, for ex- 

 ample, not only by the above general mixture, but red and blue- 

 green rays, or orange and blue, or yellow and violet, taken in pairs 

 in certain proportions, and acting simultaneously or in very rapid 

 succession on the same part of the retina, cause the sensation of 

 white: such colors are called complementary to one another. The 

 mixture may be made in several ways; as, for example, by caus- 

 ing the red and blue-green parts of the spectrum to overlap, or by 

 painting red and blue-green sectors on a disk and rotating it 

 rapidly; they cannot be made, however, by mixing pigments, 

 since what happens in such cases is a very complex phenomenon. 

 Painters, for example, are accustomed to produce green by mix- 

 ing blue and yellow paints, and some may be inclined to ridicule 

 the statement that yellow and blue when mixed give white. 

 When, however, we mix the pigments we do not combine the 

 sensations of the same name, which is the matter in question. Blue 

 paint is blue because it absorbs all the rays of the sunlight except 

 the blue and some of the green ; yellow is yellow because it absorbs 

 all but the yellow and some of the green, and when blue and yel- 

 low are mixed the blue absorbs all the distinctive part of the 

 yellow and the yellow does the same for the blue ; and so only the 

 green is left over to reflect light to the eye, and the mixture has 

 that color. Grass-green has no complementary color in the solar 

 spectrum; but with purple, which is made by mixing red and blue, 

 it gives white. Several other colors taken three together, give 

 also the sensation of white. If then we call the light-rays which 

 arouse in us the sensation red, a, those giving us the sensation 

 orange 6, yellow c, and so on, we find that we get the sensation 

 white with a, b, c, d, e, f, and g all together; or with b and e, or 

 with c and /, or with a, d, and e; our sensation white has no deter- 

 minate relation to ethereal oscillations of a given period, and the 

 same is true for several other colors; yellow feeling, for example, 

 may be excited by ethereal vibrations of one given wave-length 

 (spectral yellow) , or by a light containing only such waves as 

 taken separately cause the sensations red and grass-green; in 

 otner words, a physical light in which there are no waves of the 



