Xo. 486] 



THE PROBLEM OF COLOR VISION 



373 



which any of the colors is unknown; and the notion derived from 

 studying the color terms and references in ancient literature, 

 that man in historic times had a deficient color sense, is not sub- 

 stantiated. It may be that as in children, the red portion of the 

 spectrum was preferred to the blue, but even this is not e>tal)- 



All the colors which are normally perceived 

 may be ])ro(luce(l by combinations of the spectral red, green, and 

 blue. Normal \ ision is therefore trichromatic. Sometimes in 

 trichromatic vision the red end of the spectrum is shortened; in 

 other cases a nu'xture of red and green, which to normal persons 

 appears pure yellow, may seem tinged with red or green. Thus 

 there are variations in trichromatic vision, (ireater abnormalities 

 may take the form of dichromatic and monochromatic vision. The 

 latter is a rare pathological condition in which all colors are per- 

 ceived as shades of one; vision therefore is essentially colorless 

 (achromatic), the images obtained being comparable with photo- 

 graphs. In dichromatic vision color perception is so limited that 

 all of the shades perceived may be ma'le by combining two (.f the 

 spectral colors red, green, and blue; blindnes. to the third of the>e 

 colors may be partial or com|)lete. Tlu^ ordinary color blindness 



hereditary. It may cause so little trouble as to pa^s undetected 

 until the age of seventy. All attempts to overconie the color 

 blindness by educating the color sense in various ways, have failed. 



Since dichromatic color blindness plays so large a part in the 

 theories of normal vision, a portion of Dr. Pole's description of his 

 own case is here inserted. He says,' "In the fir-^t place we see 

 white and black and their intermediate gray. i)rovided they are 

 free from allov with other colors, precisely as others do. (Such 

 statements are conHrme.l by those who are color blin.l in one .-ve, 

 the other being normal.) Secondly there are two c.)lor^. namely 

 yellow and bhie, which also if unalloved we se,-. so far as can be 



