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THE AMERICAN NATURALIST 



[Vol. XLI 



cists recognize that only three are necessary. Accordingly the 

 physicist Young proposed a simpler theory antedating that of 

 Hering. It was advocated by Helmholtz, and is generally known 

 as the Young-Helmholtz theory. 



According to the Young-Helmholtz theory there are three photo- 

 chemical substances, red, green, and blue respectively, which are 

 stimulated by the various rays of the spectrum as shown in figure 

 4. Absence of stimulation produces black, and the simultaneous 

 disassociation of all three yields white. Protanopia is interpreted 

 as red blindness, due to deficiency of the red perceiving substance. 

 Deuteranopia is green blindness, and tritanopia is blue blindness. 

 Since it would appear that the })ercepti()n of white must be lost 



with the disa])pearan('e of one of the three elements, the theoi 

 has been variously modified, in j)n)t;nio[)ia The red and the gree 

 substances may We so iiltered that each fe^poiiiU hoth to red an 

 green light iFieki, or the vvi\ and the ui-eeii siihstances may I 

 imperfectly segregated, as assumed by Mrs. Franklin's theor 

 The close relation between the red and green substances is shov, 

 in Koenigs presentation of the Young-Helmholtz theory (Fig. 5 

 The absence of either would give rise to somewhat similar cond 

 tioiis. siieli as occur in protanopia and deuteranopia. The figui 

 iraheates that in triehroiuatie vision, the colors from vellow i 



