COLOR VISION. 521 



we may speak of these as red, yellow, blue, etc., rays; all 

 together, in about equal proportions, they arouse the sen- 

 sation of white. A remarkable fact is that most color feel- 

 ings can be aroused in several ways. White, for example, 

 not only by the above general mixture, but ^red^nd blue- 

 green rays^ or orange and blue, or yellow and violet, taken 

 together in pairs/^cause the se"nsation of white: such colors 

 are called complemmtary to one another. The mixture 

 may be made in several ways; as, for example, by causing 

 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 mix- 

 ing pigments, since what happens in such cases is a very 

 complex phenomenon. Painters, for example, are accus- 

 tomed to produce green by mixing blue and yellow paints, 

 and some may be inclined to ridicule the statement that yel- 

 low 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 hand. 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 yellow are mixed the blue absorbs all the distinc- 

 tive 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 b, yellow c r and so on, we find that we 

 get the sensation white with a, I, c, d, e, /and g all together; 

 or with b and e, or with c and /, or with a, d, and e\ our 

 sensation white has no determinate 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 mixing red and grass-green, which 



