THEORIES OF COLOUR- VISION. 1 1 19 



and the latter as relatively blue-sighted, these conditions depending 

 on relatively greater and less pigmentation respectively. Hering 

 regards the photerythrous group as relatively yellow-sighted, and 

 corresponding to this he found that Biedermann's matches were good for 

 photerythrous cases. The scoterythrous group, on the other hand, would 

 be relatively blue-sighted, and Singer's matches were found to be good 

 for them, but required alteration for Biedermann. Hering prepared a 

 number of dried human retinae, and found when Singer looked through 

 the macular region of one of these that Biedermann's matches appeared 

 correct, and by the same means Hering was able to convert a scotery- 

 throus into a photerythrous case. Hering also found that the extra- 

 macular region of his own retina gave results which stood in the same 

 relation to those of his central region as the results for the central region 

 of scoterythrous stood to those of central photerythrous vision. Bieder- 

 mann and Singer carried out a series of observations on their peripheral 

 vision, and Hering expected to find that the differences between them 

 would disappear. They remained present, however, though considerably 

 diminished in amount, and Hering ascribed this to differences in the 

 pigmentation of the lens, which he found to be of a distinct yellow- 

 green colour, not only in adults but in the newly-born child. 1 



If this explanation be correct, it might be expected that there would 

 be a gradual transition of one class into the other, corresponding with a 

 graded series of degrees of pigmentation, but the evidence seems to be 

 against this. Hering himself was inclined to think that the cases he 

 examined centred round Biedermann and Singer, rather than round a 

 point between them. It is quite possible, however, that the variations 

 of pigmentation may be discontinuous; and, in the absence of direct 

 investigation of the question, the existence of two distinct groups by no 

 means destroys the validity of the proposed explanation. 



From his examination of individual variations, it seemed probable to 

 Hering that there is a relation between macular (and lens) pigmentation 

 and development of the colour sense. Biedermann had a very highly 

 developed colour sense (high degree of discrimination and wide limits 

 of visibility of spectrum), while Singer had a weak red-green sense, and 

 much more limited spectrum ; and in cases of red-green blindness it 

 seemed as if the group with more pigmentation (yellow-sighted or 

 photerythrous) had a more highly developed blue-yellow sense, and 

 Hering supposes that the shortening of the spectrum in the scoterythrous 

 group may be due to weakness of the yellow sensation. 



v. Kries has recently written strongly against Hering's view. He points 

 out that the most marked distinction between the two groups of red-green 

 blindness is at the long- wave end of the spectrum, while the macular pigment, 

 as Sachs has shown, only begins to show any appreciable absorption in the 

 yellow-green. In order to determine the influence of macular pigmentation in 

 a number of cases of dichromatic vision, he matched blue-green light of 490 A 



with a mixture of blue (460 X) and green (510-517 X); the quotient - 



green 



showed no regular difference between the two groups of red-green blindness ; 

 in fact, the extremes were both in the scoterythrous class. 



According to Hering's theory, there should be in yellow-blue blindness two 

 neutral bands, without shortening of either end of the spectrum. Hering 



1 The greenish yellow colour of the lens in childhood is seen best if observed in water 

 placed in a white porcelain vessel, The greenish yellow colour changes to yellow in adult life. 



