2IO SECTIONAL ADDRESSES 



or seven colours. The red-green colour blind has a spectrum composed 

 of only two colours, these two colours being yellow and blue. The 

 acceptance of this fact was delayed for a long time, and it is doubtful if 

 it is yet generally accepted. Herschel in 1845 ^^^ the first to put forward 

 the dichromic explanation of colour blindness. He pointed out in his 

 article on ' Light ' in the Encyclopcedia Metropolitana that certain 

 individuals could only distinguish two colours, and that these two colours 

 were yellow and blue. Clerk-Maxwell, among others, opposed this, for 

 he accepted the theory put forward at that time by Young and Helmholtz 

 that colour blinds were either red-blind, green-blind or violet-blind, and 

 that the red-blind were blind to red, but could see the remaining two 

 colours, green and violet : the green-blind were blind to green, but could 

 see red and violet. 



In an account by Pole of his own case,^* we find he vigorously protests 

 against these prevailing beliefs, and gives a careful analysis of his own 

 colour vision as evidence. He had been pronounced red-blind by Maxwell 

 and green-blind by Holmgren. Pole repudiated both suggestions, and 

 claimed that the true solution was that he was blind to both colours. 

 This conclusion was confirmed by a large number of facts. One was the 

 evidence of the colour blinds themselves who, whether they were classified 

 as red-blind or green-blind, asserted that their colour sense was composed 

 of blue and yellow. Another was a case of congenital unilateral dichroma- 

 tism investigated by von Hippel in 1880. The individual tested had 

 normal colour vision in his left eye but was colour blind in his right eye. 

 With the right eye he confused red and green with yellow, as tested by 

 various standard tests. The colours which the subject could distinguish 

 with his right eye were blue and yellow, these being confirmed when he 

 looked at them with his normal eye. Von Hippel diagnosed the case as 

 one of red-green blindness with spectrum of normal length. Holmgren 

 examined the case with his wool test and proclaimed it to be one of 

 red-blindness with shortened spectrum in accordance with the Helmholtz 

 theory. Von Hippel retested his subject and reaffirmed his first finding. 

 This was the first case of monocular colour vision reported, and it un- 

 doubtedly strengthened the case for the acceptance of Herschel's and 

 Pole's views. Holmgren, it is interesting to note, ultimately agreed that 

 blue and yellow were the only colours seen. That this point is still 

 controversial may be gathered from the statement made by Pitt ^^ in a 

 recent investigation, that the fundamental responses of the protanope are 

 blue and a saturated green, while those of the deuteranope are blue and 

 a mixture of red and green. 



Only a few cases of unilateral colour blindness have been recorded, 

 and these, according to Parsons, are of ' doubtful value.' In these cases, 

 however, the suggestion is always that the colours seen are blue and 

 yellow. Miles and Beaumont,2° whcJ tested the two eyes separately in 



1' Proc. Roy. Soc, 1856. 



^' Med. Res. Council Report, XIV. ' Characteristics of Dichromatic Vision.' 

 Special Report Series, No. 200. H.M. Stationery Office, 1935. 



2" ' Monocular Testing of the Colourblind,' American J. Ophth., 1931, 14, 

 636-639. 



