450 DR WILLIAM POLE ON THE PRESENT STATE OF 



the absence of the red, this sensation had, in its brightest example, been transferred to the 

 wave-length usually called yellow, and therefore we naturally called it by that name. 



But, as time went on, the theoretical views changed. Adhering still to Young's 

 theory, it was found advisable not to insist on the general prevalence of red as the 

 absent sensation. It might be either of the three, and there seemed reason to believe 

 that the absence of green was also common. Helmholtz adopted this idea in his work 

 of 1867, distinguishing the two classes of defect as " Red-blindness" and "Green-blind- 

 ness " respectively, with a possibility of " Violet-blindness " also. 



The same classification was adopted by Professor Holmgren in his well-known book of 

 1877, in which he gave means, by his wool tests, of distinguishing the red-blindness and 

 green-blindness from each other. When this came to my knowledge, I entered into 

 communication with him, and sent him my wool matches, arranged according to his 

 directions. He courteously examined them, and gave me a positive answer that I was 

 " green-blind," and not " red-blind " as Maxwell and all others had supposed.* 



According to this, therefore, my warm sensation would be produced by the red 

 fundamental ; but as I suppose Maxwell's explanation of the shifting of the maximum 

 intensity would also apply here, I should still be justified in calling it yellow. 



It will be well now to explain how the testimony of the colour-blind patient is given. 

 I have alluded to the difficulty of describing what the subjective sensation of colour is ; 

 but it must be recollected that this difficulty is not peculiar to the colour-blind ; it applies 

 alike to the normal-eyed in their communications with each other. How is a normal- 

 eyed person A to know that his neighbour B has the same kind of vision as his own ? 

 It can only be by verbal comparisons of impressions. A will exhibit to B some samples 

 of colours, and will ask his impressions about them ; and if B's answers show that his 

 ideas about redness, greenness, blueness, and so on, accord with those of A, he will be pro- 

 nounced normal-eyed. Brit if he were to examine me in the same way, he would find 

 my answers show ideas so different from his, that he must consider my vision defective 

 and abnormal. 



Now, let us carry this a little farther. Suppose I tell my examiner that I am aware 

 I have only two colour-sensations, a warm and a cold one, but that I am not sure what 

 sensations of his they correspond to, and I wish him to aid me in finding this out, that I 

 may know what to call them. 



He might probably begin by showing me the most common colour in nature — green. 

 He would point to fresh grass or young leaves of plants, and ask me what I thought 

 of them ? I should say they came within my warm colour, but were only dull and dingy 

 specimens of it. An emerald-green paper I should characterise similarly, and a Bruns- 

 wick green I should pronounce still darker, with scarcely any colour at all. He would 

 show me some powerful green signal-glasses, which I should say belonged to my opposite 

 or cold colour. He might ask me to look at the middle of the spectrum, containiug what 



* See, however, further explanations of Holmgren's general views on pp. 453, 454, 455, and 461. 



