DICHROMATIC VISION 169 



variation, which is to be explained on physical grounds. This is the 

 part of the spectrum where macular pigmentation makes itself most 

 felt. Moreover, here too adaptation is most likely to introduce 

 complexity. It is probable that there is no crucial difference between 

 the blue stimulus values of the two groups. 



Macular pigmentation also probably accounts for the difference 

 between the two red curves (Fig. 45) beyond 552 /x^u,. The ratios of 

 the red of one observer to that of the other throughout the spectrum 

 agree well with the determination of the absorption of the yellow pig- 

 ment by Sachs. Similarly the ratio of blue to red in two persons 

 gave the same value (0'3) as for the normal (v. Kries). The estimation 

 of macular absorption is much easier in dichromats than in normal 

 trichromats. 



Macular pigmentation is a serious complication in the accurate 

 determination of the neutral point. This point, where the mono- 

 chromatic colour appears colourless or white to the dichromat, should 

 be situated w^here the W- and /i-curves intersect, and hence it should 

 be situated rather nearer the violet end in protanopes than in deuter- 

 anopes. If, however, there is much macular absorption the light will 

 look yellower and the neutral point will be displaced slightly towards 

 the red end. Indeed, the neutral point of a protanope with densely 

 pigmented macula may be nearer the red end than that of a deuteranope 

 with slightly pigmented macula, v. Kries has made the following 

 estimates, specially directed to elucidate this matter. 



Wave length of homogeneous match 

 Nature of light 



Daylight — reflected from magnesium oxide coated sur- 

 face 

 Light reflected from clouds — weakened by ground glass 

 :, „ „ „ weakened by smoked glass 



These estimates were made with a comparison white light. 



The existence of a neutral point shows that dichromats receive 

 the same sensory impression from what normal colour-sighted indi- 

 viduals call white, and also from a certain homogeneous spectral light 

 and from the mixture of red and blue which to them matches that 

 homogeneous light. The homogeneous light appears to the normal a 

 highly saturated bluish-green ; the red-blue match appears a red- 

 purple. If we suppress nuances it may therefore be said that one of 

 the chief characteristics of both groups of dichromats is that they 

 confuse red and green. But this is a very imperfect expression of 



