THE EYE AS A SENSORY APPARATUS. 545 



number of primary color sensations, and that the rest are due 

 to combinations of these. That a compound color sensation 

 may be very different from its components when these are, 

 regarded apart, is clearly shown by the sensation white 

 aroused either by what we may call red and blue-green, or 

 green and purple, stimuli acting together; or of yellow due 

 to grass-green and red. To account for our various color sen- 

 sations we may, therefore, assume a much smaller number of 

 primary sensations than the total number of color sensations 

 we experience; all can in fact be explained by assuming any 

 three primary color sensations which together give white, and 

 regarding all the rest as due to mixtures of these in various 

 proportions; there may be more than three, but three will 

 account for all the phenomena, black being a sensation expe- 

 rienced when all visual stimuli are absent. This is known as 

 Young's theory of color vision, and is that at present most 

 commonly accepted. The selection of the three primary sen- 

 sations is decided by the phenomena of color-blindness, which 

 show that if this theory of color vision be correct red must 

 be one of the primary color sensations: if so, then green 

 and violet must be the other two. The theory further 

 assumes that all kinds of light stimulating the end appa- 

 ratuses give rise to all three sensations, but not necessarily in 

 the same proportion. When all are equally aroused the sen- 

 sation is white or some shade of gray when the red and green 

 are tolerably powerfully excited and the violet little, the sen- 

 sation is yellow; when the green powerfully and the red and 

 violet little, the sensation is green, and so on. In this way 

 we can also explain the fact that all colored surfaces when 

 intensely illuminated pass into white. A red light, for ex- 

 ample, excites the primary red sensation most, but green and 

 violet a little; as the light becomes stronger a limit is 

 reached beyond which the red sensation cannot go, but the 

 green and violet go on increasing with the intensity of the 

 light, until they too reach their limits; and all three primary 

 sensations being then equally aroused, the sensation white is 

 produced. 



Color Blindness. Some persons fail to distinguish colors 

 which are to others quite different; when such a deficiency is 

 well marked it is known as "colorblindness," and, assuming 

 Young's theory to be correct, it may be explained by an ab- 

 sence of one or more of the three primary color sensations; 



