454 DR WILLIAM POLE ON THE PRESENT STATE OF 



the different Colours ?" and the extracts in " Data," letter G, will show how conclusive 

 the author considered the case. 



Another great authority on Colour- Vision, Professor Von Kries, also thought so 

 much of this case that he published some remarks on it, noticed in " Data," letter H. 



It is worthy of mention that both the last-named authorities have been, and still 

 are, staunch supporters of the Young-Helm holtz theory of colour- vision, and do not 

 admit that these facts affect its validity ; but they agree that the application of it to 

 explain dichromatism by the loss of one of the fundamental sensations is no longer 

 tenable. 



Other similar cases were afterwards found and examined by Holmgren, who gave 

 a full description of them at the International Medical Congress at Copenhagen in 1884, 

 when he produced diagrams of the colour-sensations. 



In 1890 Professor Hering investigated thoroughly a case of the kind with apparatus 

 specially prepared for the purpose. The result was precisely similar ; the yellow, blue, 

 and white suffering, in the dichromic eye, no change of hue. See " Data," letter I. 



In the same year Dr Hess of Leipsic found a singular case of a young man who 

 was affected with congenital dichromatism in one-half of the retina of one eye, the effect 

 being the same. See " Data," letter J. 



These accumulated proofs furnish, perhaps, the most direct and the strongest evidence 

 in favour of the yellow and blue. 



TJie Dichromic Zone of the Normal Retina. 



But the most remarkable fact regarding the comparison of Dichromic with Normal 

 vision is, that the opportunity for observing it exists in all normal eyes. 



The normal vision does not extend over the whole retina ; it is confined to a central 

 portion of it ; round this there is an annular zone, in which the vision is dichromic. 

 Here, as Helmholtz describes it, "the distinction between yellow and blue stands 

 prominently out, but saturated red appears nearly black, or dark yellow-brown ; — and 

 leaf-green is a yellowish- white." (This, it will be at once seen, corresponds perfectly 

 with dichromic vision.) Kound this dichromic zone there is a third space occupying 

 the periphery of the retina, where no colours are visible, objects appearing grey, 

 with only variations of light and shade. (This corresponds with absolute or total 

 colour-blindness.) 



The general fact of this peculiarity has been long known. In 1828 Purkinje 

 remarked that certain colours appeared changed in parts of the retina away from the 

 centre. Aubert investigated the matter in 1857, and established the general facts 

 above stated, at the same time noticing the analogy between them and congenitally 

 defective colour-vision. 



Schelske, in 18G3, went more into detail, testing the colours with the aid of 



