KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 459 



nomenclature is founded, is abandoned, the classification falls with it, at least so far as 

 its assigned cause is concerned. But as it is certain that the variability which prompted 

 the division does actually exist, it is necessary to devote attention to it, and to show how 

 it has been treated by various writers. 



In the first place, it may be useful to inquire how, under the supposed classification, 

 the difference between red and green blindness was tested and defined ? One mode was 

 suggested by Preyer, 1868, who said, " If anyone who confounds red with green sees the 

 spectrum clearly shortened at the red end, he is red-blind; if not, he is green-blind." 

 But Preyer (p. 322) seems to have decided from my symptoms, as Maxwell and 

 Herschel did, that I was red-blind, although I see the red end of the spectrum perfectly. 

 Another kind of test is to ask the patient to select matches to a skein of moderately 

 bright red wool ; and if he is really colour-blind, some of these will probably be green 

 or brown, or both. Then, if these appear dark to the normal eye, he is said to be red- 

 blind ; if brighter, green-blind. Helmholtz said " both classes confuse the red hue 

 with green, but the green-blind choose a yellower green than the red-blind. Konig, 1883, 

 defined that the so-called " red-blind matched a bright red (helles Both) with a dark green 

 (dunMes Grihi), while the so-called green-blind matched a dark red with a bright 

 green. 



But there is nothing in these tests to show blindness to red, or blindness to 

 green, unless theory had suggested the idea beforehand. AVhat they do show, by 

 independent interpretation, is a much simpler thing, namely, the existence of variable 

 degrees of impressibility by rays of certain wave-lengths, as will be fully explained and 

 illustrated further on. 



Professor Maxwell gave a much more scientific method of determining the 

 missing colour by means of Newton's diagrams. In his Paper of 1860 he drew 

 (fig. 11) a diagram applicable to a case of dichromic vision examined by him, from which 

 he inferred that the point D represented " that colour, the absence of which constitutes 

 the defect of the dichromic eye," and he said " it agrees pretty well with the colour 

 which Mr Pole describes as neutral to him, though crimson to others." I have drawn a 

 corresponding diagram for my own vision, and find it gives a precisely analogous red 

 point, D, only rather more tending to purple. According to this, therefore, Mr Maxwell's 

 friend and I were both classed as " red-blind." 



But how is this to be reconciled with the later opinion, a very positive one, not only 

 of Professor Holmgren, but of other authorities, that I am " green-blind " ? Am I 

 to commit the irreverence of saying that Professor Maxwell was wrong ? Far from it. 

 He appealed to me as giving practical testimony to the correctness of his reasoning, and I 

 cheerfully renew it now. His diagram does undoubtedly prove that I am red -blind, 

 exactly as he says. Neither, on the other hand, do I question the correctness of Holmgren 

 and others who pronounced me green-blind : they were right too. The true solution is 

 that I am blind to both colours. 



