452 DR WILLIAM POLE ON THE PRESENT STATE OF 



generally, should carry much weight, as he is in a far better position to know what his 

 own impressions are than any normal-eyed person can be. In my own case it is hardly 

 presumptuous to say that I must have acquired some notion how to compare the sources 

 of information. And when I say that my long experience and observation have fully 

 convinced me that I have no idea whatever of the colour-sensations which the world call 

 red and green, but that my two ideas of colour correspond to what the world call yellow 

 and blue, I think my assertion is worthy of attention. 



B. Evidence furnished by Direct Comparison with Normal Vision. 



But Professor Maxwell might say, " I admit the truth of your testimony, that your 

 warm-colour sensation is the same as that given you by objects called yellow ; but my 

 theory says that, owing to the defect of your vision, these objects give you the sensation 

 of green or red." To answer this we must turn to the second part of our argument, which 

 is, that the evidence in favour of yellow and blue, as the true dichromic sensations, does 

 not rest solely on the testimony of the colour-blind. 



When the collision between the testimony and the theory became more known, foreign 

 physiologists bestirred themselves to get further light thrown on the matter, and, by 

 industrious research, at length discovered means by which a direct comparison could be 

 made between the normal and the dichromic sensations. Colour-blind persons could not, 

 indeed, be made to see normally, but there did seem a possibility that the converse 

 process might be made available. Could any cases be found where a person, having 

 experienced normal vision, might assume the vision of a dichromic patient, and so be 

 able directly to compare the sensations with each other ? By industry and perseverance 

 it was found that this could be effected in several ways. 



Dichromic Vision acquired by Disease. 



In the practice of oculists, it was found that certain affections of the eye were 

 attended by irregularities in the vision of colours ; and, in 1862, Dr Benedict of Vienna 

 remarked that many of these cases resembled congenital dichromic vision. 



Schelske, in 1865, noted a similar case, which he tested with Maxwell's colour 

 discs, proving a perfect dichromatism with the colours yellow and blue, which, of course., 

 the patient knew well in his healthy state. See further particulars in the " Data," 

 published in vol. xx. of the Proceedings of this Society ; letter A. 



It was afterwards found that such cases were often brought on by undue indulgence 

 in alcoholic stimulants, and by the use of tobacco. In 1878 Professor Nuel of Liege 

 published a paper called "1/ Amblyopic Alcoolique et le Daltonisme," in which he 

 described a " scotome centrale " produced in this way. It showed a temporary change of 

 the action of a portion of the retina, producing a state corresponding to colour-blindness ; 



