CASE OF COLOUR BLINDNESS. 507 



Hering proved that this case was a distinct case of blindness to yellow and blue 

 with a neutral point in the yellow part of the spectrum and another in the blue part. 

 He also proved that, though the light adjacent to this second neutral point was termed 

 colourless, the light on the less refrangible side possessed a green valency, while the light 

 on the more refrangible side possessed a red valency. 



In Mr A.'s case, the reduction of intensity, if it occurs at all, is so slight that the 

 spectrum is visible to its full extent. The red part is visible over its whole normal 

 range, and no colour is visible in any other portion. 



Conclusion. 



Mr A.'s case is, strictly speaking, one of blindness to all colours except red. Yet it 

 is not a case of monochromasy. It seems to be strictly a case of dichromasy, for all 

 the tests show that there is a true neutral point in the yellow part of the spectrum, and 

 that the colourless light of the green and blue parts is complementary to the red. 

 I cannot certainly say as yet that there is no second neutral point in the blue part of 

 the spectrum, but I have not got the slightest evidence of it or of the existence 

 of a red valency in the violet region. Yet Mr A. seems to possess the full normal 

 sensitiveness to red. He can detect the existence of red in white or black apparently 

 quite as soon as an observer who possesses normal vision can. He can distinguish the 

 various shades of red and name them correctly — marone, scarlet, strawberry, &c, and 

 even pink. Of course, in the latter case, he might be deceived by a red mixed with 

 grey instead of blue. Yet, as 1 have stated, he has never shown any tendency to 

 speak of the violet light of the spectrum as red. Still, the point is not decided, and 

 I intend to investigate it further. 



The fact that the violet end of the spectrum is unshortened precludes the case from 

 being described as the " violet-blindness " of the unmodified Young-Helmholtz theory. 

 Yet the neutral point in the yellow is in agreement with that description. In " partial 

 green blindness," on this theory, the spectrum has its full length, and, though a large 

 part of the green portion may be grey, the blue is visible. The immense differences 

 in the appreciation of red and green, laying aside the question of a second neutral point, 

 tell against the application of Hering' s theory. In any case, the modern modification 

 of the Young-Helmholtz theory can apply. I have already given a discussion 

 of the bearings of the case on the theories, but I defer its publication until the 

 experimental investigation is completed. 



It is interesting to compare the case with the only genuine case of " violet-blindness " 

 which Captain Abney has met (up till 1895). In the latter, the spectrum was shortened 

 at the violet end and there was a neutral point in the yellow part. Only two colours 

 were named — red and black Green was called " bright-black," though it did not 

 satisfactorily match a darkened white patch. Blue was called " dark-black." Suppose 



