THL EYE AS A SENSORY APPARATUS. 543 



into white. Of all the colors of the spectrum yellow most 

 easily passes into white with strong illumination. Black and 

 white, with the grays which are mixtures of the two, thus 

 seem to stand apart from all the rest as the fundamental 

 visual sensations, and the others alone are in common par- 

 lance named " colors." It has even been suggested that the 

 power of differentiating them in sensation has only lately 

 been acquired by man, and a certain amount of evidence has 

 been adduced from passages in the Iliad to prove that the 

 Greeks in Homer's time confused together colors that are 

 very different to most modern eyes; at any rate there seems 

 to be no doubt that the color sense can be greatly improved 

 by practice; women whose mode of dress causes them to pay 

 more attention to the matter, have, as a general rule, a more 

 acute color sense than men. 



Leaving aside black, white, gray, and the various browns 

 (which are only dark tints of other colors), we may enumer- 

 ate our color sensations as red, orange, yellow, green, blue, 

 violet, or purple; between each there are, however, numerous 

 transition shades, as yellow-green, blue-green, etc., so that 

 the number which shall have definite names given to them is 

 to a large extent arbitrary. Of the above, all but purple are 

 found in the spectrum given when sunlight is separated by a 

 prism into its rays of different refrangibility; rays of a cer- 

 tain wave-length or period of oscillation cause in us the feel- 

 ing red; others yellow, and so on; for convenience we may 

 speak of these as red, yellow, blue, etc., rays; all together, in 

 about equal proportions, they arouse the sensation of white. 

 A remarkable fact is that most color feelings can be aroused 

 in several ways. White, for example, 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 propor- 

 tions, and acting simultaneously or in very rapid succession 

 on the same part of the retina, cause the sensation of white: 

 snch colors are called complementary 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 

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

 complex phenomenon. Painters, for example, are accustomed 

 to produce green by mixing blue and yellow paints, and some 



