STATEMENT OF THE THEORY 259 



should be the same for the totally colour-blind and for the extreme 

 periphery of the retina, which is not the case (v. Kries) {v. p. 71). 

 Dichromatic vision is attributed to insensitiveness or absence of the 

 red-green substance, or of the yellow-blue substance (tritanopia). 



The two classes of red-green blindness, protanopia and deuteranopia, 

 offer serious objections to Hering's hypothesis, and these have not yet 

 been overcome satisfactorily. His original explanation was that the 

 differences were due to physical causes, especially macular pigmentation, 

 and the same was applied by him to the variations in the Rayleigh 

 match (anomalous trichromatic vision). Hering^ very carefully ex- 

 amined Prof. Biedermann and Dr Singer, well-marked examples of indi- 

 vidual variation in pigmentation. He regarded the former as relatively 

 yellow-sighted, with little macular pigmentation, the latter as relatively 

 blue-sighted, with greater pigmentation. He found Biedermann's 

 matches agreed with deuteranopic matches, Singer's with protanopic. 

 Experiments in which the subjects looked through the macular region 

 of dried human retinae were held to give confirmatory results. His 

 own extrafoveal matches compared with his fovea] matches were of 

 the same nature as the deuteranopic to the protanopic foveal matches. 

 On the other hand Biedermann and Singer's peripheral matches did not 

 agree, as should have been the case, but the differences were diminished 

 in amount, and were attributed to absorption by the crystalline lenses. 



V. Kries {v. p. 168), Abney and others have shown conclusively that 

 the differences between protanopic and deuteranopic vision cannot be 

 explained by any such physical causes. 



Some of Hering's assumptions are difficult to accept. The absence 

 of any direct evidence of anabolic processes acting as physiological 

 stimuli has already been mentioned, but it is not a serious objection. 

 The balance between assimilation and dissimilation is obviously com- 

 parable to the balance between excitation and inhibition^, and analogies 

 can be readily found in Sherrington's researches. 



It is, however, a distinct objection that the theory demands twice 

 as many variables as the results of colour-mixtures demand. Brunner^ 

 has attempted to eliminate this difficulty by ascribing to each substance 

 a reversible photochemical process*. 



1 Lotos, VI 142, 1885. 



2 Cf. Bernstein, Naturwissenschaftliche Rundschau, xxi. 497, 1906. 



3 Arch. J. d. ges. Physiol, cxxi. .370, 1908. 



* See also Pauli, Der kolloidale Zustand u. die Vorgdnge in der lebendigen Substanz, 1902. 

 For examples of reversible photochemical processes, see Stobbe, Ann. d. Chemie, cccux. 1, 

 1908. 



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