Photoreception 



437 



ment which is somewhat different from that used by normal individuals. 

 These individuals are said to have an anomalous trichromatism or to have a 

 "color weakness," i.e., a weak sensitivity to certain colors. Other indixiduals 

 can match the spectrum with onlv two colored lights; thev are said to be 

 dichromatic or color blind. Other individuals are not able to detect any 

 chromatic differences in the spectrum; they are completely color blind. 



Observers with dichromatic vision are of several types. One type, the 

 deuteranope, can distinguish yellow from blue but not green from red, and 

 sees both red and green as shades of grav. Another tvpc, the protanope, is able 

 to distinguish blue from yellow but not green from red, and, in addition, is 

 less sensitive to red than a normal person or a deuteranope, i.e., he sees red 

 not as various shades of grav, as does the deuteranope, but as a very dark 

 gray or black. A third and rare type, the tritanope, is able to distinguish be- 

 tween green and red but not between yellow and blue. 



Color blindness of the first two types, deuteranopia and protanopia, and 

 their respective color weaknesses, deuteranomaly and protanomaly, are in- 

 herited as sex-linked characters and are therefore about twenty times as 

 common in men as they are in women (total abnormality for men, 8.0 per 

 cent; for women, 0.43 per cent). 



The Peripheral Mechanism of Color Vision. It was suggested by Thomas 

 Young in 1807 that the fundamental mechanism of color vision was a dif- 



•400 -450 -500 



Fig. 141. The sensitivity curve (open circles) of the receptors connected with a single 

 ganglion cell of the dark-adapted retina of a cat. The black dots represent the absorption 

 of light by visual purple. U, Experimentally obtained curve from which curve P (in 

 proper magnitude, p) is substracted to give the sensitivity curve of specific color receptors. 

 From Granit." 



ferential sensitivity of the individual receptors. The theory was expanded by 

 Helmholz in 1852 and finally, after the publication of more than 1200 papers 

 by many investigators during a period of almost a century and a half, the 

 supporting experimental data were supplied for an invertebrate eye by 

 Graham and Harthne^** and for the vertebrate eye by Granit.^'' 



