KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 455 



Maxwell's colour-top; by which he obtained equations prepisely corresponding to those 

 of dichromatism. I have given these in " Data," letter K, and reduced them to a 

 diagram, which may be compared with that of my own vision, in the Phil. Mag. 



In 1874 Professor Fick of Wiirzburg treated fully of this subject ; and again, in 1879, 

 more positively asserted the identity of this portion of the retina with that of the colour- 

 blind. See " Data," letter L. 



Professor Holmgren remarks, in 1878 (German translation, page 4), on this subject as 

 follows ; and, as he is always quoted as a strong supporter of Young's theory, his 

 opinion is weighty : — 



The intermediate zone of the normal field of vision is of very special interest, because it gives 

 us the opportunity of seeing, with our own eyes, how the red-blind see, and thus of becoming, in the 

 most direct way, acquainted with their abnormal impressions. We see in this band only yellow and 

 blue, and convince ourselves that the red-blind not only designate yellow and blue as principal 

 colours, but also see them in the same way as the normal eye. 



He states that he considers this may be accounted for theoretically, and adds : — 



The theory drives us to the assumption that the excitation of the green and violet elements 

 gives rise, under certain circumstances, as with the red-blind, to white, and not to green-blue ; — and 

 that the excitation of the red and violet elements in certain cases, as with the green-blind, produces 

 not purple but white. And, according to this, we must also concede that the excitement of the 

 green element causes the sensation of yellow when the complementary of yellow, i.e. blue, is present. 

 The excitement of the green organ only calls up the sensation of green on a retina which possesses 

 the red-sensitive element. 



In 1888 Von Kries devoted a section of his work of that date to a full investigation 

 of this point. See " Data," letter M. 



A still more thorough investigation of the same matter has been made within the last 

 year or two by Dr Hess of Leipsic, a notice of which will be found in " Data," letter 

 N. It will be seen that it amply confirms and establishes the two relevant points in this 

 section of our inquiry, namely, 



1. That in the dichromic zone the only colours which remain permanent and retain 

 their normal character are two definite and complementary hues of yellow and blue. 

 All other normal colours change and merge into these. 



2. That the normal white remains unchanged in all the zones of the retina. 



On the White of Dichromic Vision. 



We have now seen that there are three modes by which the dichromic sensations can 

 be actually and directly compared with the normal ones. In all these the dichromatism 

 gives every sign of exact correspondence with the congenital form ; and in all it is 

 proved that the two colour- sensations correspond with the normal yellow and blue, 

 thereby confirming the natural practical testimony of the dichromic patients. 



