COLOR BLINDNESS. 523 



assumed 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 arc equally aroused the 

 sensation is white; when the red and green are tolerably 

 powerfully excited and the violet little, the sensation 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 

 example, 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 pri- 

 mary 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 de- 

 ficiency is well marked it is known as " color blindness," 

 and, assuming Young's theory to be correct, it may be ex- 

 plained by an absence of one or more of the three primary 

 color sensations; observation of color-blind persons thus 

 helps in deciding which these are. The most common 

 form is red color blindness; persons afflicted with it con- 

 fuse reds and greens. Red to the normal eye is red because 

 it excites red sensation much, green some, and violet 

 less; and a white page white, because it excites red, green, 

 and violet sensations about equally. In a person without 

 red sensation a red object would arouse only some green 

 and violet sensation and so would be indistinguishable from 

 a bluish green; in practice it is found that many persons 

 confound these colors. Oases of green and violet color 

 blindness are also met with, but they are much rarer than 

 the red color blindness or "'Daltonism." 



The detection of color blindness is often a matter of con- 

 siderable importance, especially in sailors and railroad 

 officials, since the two colors most commonly confounded, 

 red and green, are those used in maritime and railroad 

 signals. Persons attach such different names to colors that 



