472 DR WILLIAM POLE ON THE PRESENT STATE OF 



of the existence of dichromatism may be found, in which it is no longer necessary that 

 one of the fundamental colours should be missing. He argues that it is consistent with 

 Young's theory that the three colour-sensations may still remain in force, but that the 

 two dichromic colours may be formed from them, i.e., may be compound colours. This 

 would be quite consistent with the union of the red and green forming one of the sensa- 

 tions, while the blue remained alone for the other. And it would also account for the 

 light being normal white, and for the two colours being complementary. 



In the course of Von Helmholtz's description of his new theory, he takes occasion to 

 point out that it does not admit of the former division of the colour-blind into two 

 sharply- defined classes, which indeed, he says, does not seem to be entirely confirmed by 

 observation. This decision strengthens his former verdict on the " old" explanation, by 

 removing another troublesome theoretical stumbling-block out of the way. 



Putting together the whole of the new views that are to be traced in Helmholtz's 

 second edition, we may, I think, fairly formulate his present opinions as follows : — 



1. That the original mode of explanation of colour-blindness by Young's theory is 

 essentially withdrawn, as no longer consistent with modern knowledge. The universally 

 concurrent testimony that the ordinary colour impressions of dichromic vision correspond 

 generally with the normal yellow, blue, and white, is no longer disputed. And although 

 there are variations of sensation in regard to red and green, the old ideas of blindness to 

 red and to green, as separate and contrasting defects, are abandoned. 



2. That Young's general theory of three fundamental colour-sensations is still 

 adhered to, but that the colours are now believed to differ considerably from those in 

 the spectrum. 



3. That dichromic vision may exist consistently with the retention of all three 

 fundamentals. 



That these views of an authority so eminent must exert a great and beneficial 

 influence on the general state of opinion regarding colour-blindness is, I should think, 

 indisputable. 



V. 



SUMMARY OF CONCLUSIONS. 



Let us now try to sum up the present state of our knowledge of the phenomena of 

 red-green blindness, according to the facts proved by the best investigations, and the 

 interpretation of them by the best authorities. 



1. In the first place, we arc happily freed from the long-standing incubus of the 

 " old " explanation that dichromic vision is due to the absence of one of Young's three 

 fundamental colour-sensations, and the active presence of the other two. 



