THE EYE AND THE PHYSIOLOGY OF VISION. 535 



The seven colors of the spectrum are usually spoken of 

 as the fundamental colors, because by the mixture of two 

 or more of these it is possible to produce all the other 

 colors. Thus the proper combination of all seven forms 

 white; the absence of all of them gives the perception 

 black. Black is in light what a rest is in music, but while 

 in the ear a rest is perceived as the absence of any positive 

 sensation, in the eye an absence of all light is interpreted 

 as a positive sensation, and so black seems to be a definite 

 color. By mixing any of the fundamental colors with white 

 we get the varying tints of these colors ; by mixing them 

 with black the corresponding shades. Thus red and white 

 make pink, red and black brown; green and white produce 

 light green, green and black olive, blue and white the light 

 blues, blue and black the blue blacks. Red and blue form 

 purple, purple and green produce white. Mixing colors 

 near each other in the spectrum produces an intervening 

 color. Thus red and yellow form orange. 



It must be especially borne in mind that in all instances 

 of the combinations just given the sensations are mixed and 

 not pigments. Thus on a color disk partly red and partly 

 blue the color purple will arise upon a rapid rotation of 

 that disk. If one should mix pigments quite a different re- 

 sult would occur. To mix red and blue combined in cer- 

 tain proportions produces not purple at all, as when 

 sensations are mixed, but produces green, the explanation 

 of which will be given further on. 



5. Complementary Colors. Two colors (sensations) 

 which when mixed together produce white, are spoken 

 of as complementary colors. For each color of the spec- 

 trum it is possible, if the color purple be added, to find a 

 second color complementary to it. The relations of these 

 colors are given in Figure 164, where the colors of the 

 spectrum are so arranged that those directly opposite each 

 other from the point called white are complementary. The 

 use of indigo or blueing in the process of laundering con- 



