458 DR WILLTAM POLE ON THE PRESENT STATE OF 



III. 



THE VARIATIONS IN DICHROMIC VISION. 



We now come to consider another point which has been subject to much dispute, 

 namely, the nature of the variations which are found in yellow and blue dichromic 

 vision. 



Seebeck was probably the first to point out, in 1837, that the mistakes made by 

 several persons, all suffering from the malady, presented differences of character. He 

 tried to reduce them to some general order, and he arranged them, though not ver)' - 

 definitely, in two classes. The first class, he said, had a very defective feeling for red, 

 and for what necessarily depended thereon, its complementary, green, inasmuch as they 

 scarcely distinguished these two from grey ; but their sense of yellow appeared to be 

 highly educated. His second class was not much dissimilar from the first ; they also 

 recognised yellow the best, and distinguished red somewhat better ; but they had 

 generally a weakened sensation for the less refrangible rays. Seebeck knew nothing 

 of the dichromic theory, which only arose long afterwards, but he brought out some 

 points of permanent interest. In the first place, he observed the general recognition of 

 yellow ; secondly, he noticed variations in the perception of red ; and thirdly, he con- 

 nected these with variations in the green ; all peculiarities showing at that time careful 

 observation. 



Some later writers improved on Seebeck's ideas by imagining that there were almost 

 as many varieties of defects as there were individuals affected, so that no classification 

 was possible ; but the discovery of dichromatism put an end to this notion, and gave a 

 true scientific basis to the inquiry. 



I was, I believe, the first to start on this basis, but I soon found that although the 

 new principle had swept away the great diversity of symptoms, yet it still admitted of 

 some minor variations in different individuals. I made some experiments to ascertain 

 the nature of these, and shall have occasion to discuss them in due course. Meantime, I 

 may return to the work of others. 



The explanation of colour-blindness by the Young-Helmholtz theory of vision 

 introduced, as already stated on p. 450, the division of dichromic patients into two 

 classes, called " red-blind " and " green-blind " respectively, by the assumed absence of 

 the red or the green fundamental sensation. This classification is sometimes attributed 

 to Seebeck, but there is no evidence that he had any such idea. His observations, how- 

 ever, on the variable sensitiveness to red encouraged the distinction, and subsequently 

 those who showed a diminished sensitiveness to red were called " red-blind," those who 

 did not show it were called " green-blind." 



Now, as we have seen that the old explanation of colour-blindness, on which this 



