400 DR WILLIAM POLE ON THE PRESENT STATE OF 



Maxwell's diagram proves this, by the very same reasoning that convinced 

 him I was red-blind. The line from black to white, representing a streak of 

 colourless grey, passes not only through the crimson point on the line VU, but also 

 through a blue-green point on the line UG, and both those points are colourless 

 to me. 



We may now turn to the investigations by other parties on the subject of variations 

 in dichromic vision, and the opinions they have formed thereon. 



Rose, in 1860 and 1865, published somewhat elaborate observations on what he called 

 Farbenirrsinn, describing it as the " vision of only one pair of colours complementary to 

 each other." He examined carefully 59 patients and found that no two of them 

 exhibited precisely the same sense of colour, so that no theoretical classification appeared 

 possible. 



Professor Herrmann Cohn, in 1877, examined 3490 young persons in Government 

 schools; and 100 of these, being found with defective eyesight, were subjected to 

 rigorous further tests. A summary of his results is given in " Data," letter O, the most 

 important conclusion being that blindness to red and blindness to green always went 

 together. 



In 1879 Messrs Von Kries and Kuster published a remarkable series of observations 

 with the spectrum on the variations in colour-blindness. They noted the quantity of 

 indigo (F \ G) which had to be mixed with a given quantity of red (C) to neutralise it, 

 and then determined the quantity of green-neutral (501 '5) sufficient to match the 

 mixture. The results showed variations very wide and irregular. 



In 1880 the Ophthalmological Society of Great Britain appointed a Committee to 

 investigate the subject of colour-blindness. Some extracts from this Report are given in 

 " Data," letter P, from which it will be seen that they did not favour the distinction 

 between red and green blindness. 



Professor Donders gave much attention to the variations of colour-blindness in his 

 publications of 1881 and 1884 ; and some particulars of his results are given in "Data," 

 letter Q. The general result was that the variations tended to form two marked classes, 

 which he called red-blind and green-blind for convenience, without however giving the 

 names the theoretical meaning they implied. He considered the difference between 

 them not one of kind, but only one of degree. 



In 1882 M. Van der Weyde, a pupil and assistant of Donders, undertook to investi- 

 gate the " intensity curves" of colour-blind vision. This mode of expressing the nature 

 of the vision had been first adopted by Professor Maxwell in his paper of 1860; his 

 fig. 9 showing the intensity curves of the warm and cold sensations of one patient whose 

 vision he investigated. M. Van der Weyde's object was to do this for two persons 

 chissed as red-blind and green-blind respectively (he himself being one), so that 

 they might be compared together. The result was that the curves were very 

 similar in form, but that the curve for the red-blind was simply transferred a 



