T36 iiEFOET— 1892. 



gTeenisli tinge like that of a lemon, but in other respects the observations con- 

 firnjed Pole's account of bis own case. 



Hippel's case seems to me important for another reason. By some it is believed 

 tbat congenital colour defect is due to the brain. If there had been defective 

 colour sense on one side of the brain, it would not have implicated the whole of 

 one eye, but the half of each eye. Its limitation to one eye, therefore, seems to 

 me to suggest that the fault was in the eye rather than in the brain. 



Another interesting fact in this relation is that in every normal eye, just be- 

 hind the peripheral zone of total colour-blindness, to which I have already referred, 

 there is a narrow zone in which red and green sensations are entirely wanting, 

 ■while blue and yellow sensations are normal. Possibly the red-green defect is 

 due to an imperfectly developed colour sense in the portion of the vision centre 

 «onnected with that zone of the retina, but Hippel's case seems to me to show 

 that such defect might be in the retina. 



It has probably already struck you that red-green blindness is really blindness 

 to red, green, and violet, that Young's three primary sensations appear to be 

 absent, and the two remaining colours are those which he regarded as secondary 

 compounds of his primaries. 



That, however, is not all that is revealed by colour-blindness. There is at least 

 another well-known though rare form in which a sense of yellow, blue, and violet 

 is absent, and the only colour sensations present are red and green. The defect is 

 ■sometimes termed violet blindness, but the term is somewhat misleading. It is 

 much more in accordance with the fact to term it yellow-blue blindness ; indeed, 

 we would define it precisely by terming it yellow-blne-violet blindness. Holmgren ' 

 has recorded a unilateral case of that defect analogous to Hippel's case of uni- 

 lateral red-green defect ; we therefore know definitely how the spectrum appears to 

 such a person. In the case referred to all the colours of the spectrum were seen 

 with the normal eye, but to the other eye the spectrum had only two colours, red 

 and green. The red colour extended over the whole left side of the spectrum to a 

 neutral band in the yellow-green, a little to the right of Fraunhofer's line D. All 

 the right side of the spectrum was green as far as the beginning of the violet, 

 where it ' ended with a sharp limit (about the line g).' 



If you turn to the Report of the Royal Society's Committee^ on colour 

 vision, you will find the spectrum as it appears to yellow-blue-violet blind per- 

 sons. The plate agrees with the description of Holmgren's case already given ; 

 but you will not find a representation of the spectrum as it appears to those 

 who are red-green blind, and as described by Pole and others. In place of 

 it you will find two dichromic spectra, one with a red and blue half, said to be 

 seen by a green blind, the other with a green and a blue half, said to be seen 

 bv a red blind person. We have copied the spectra for your inspection, and you 

 will observe that yellow does not appear in either of them. I do not for a 

 moment pretend to criticise these spectra from any observations of my own ; I am 

 aware Holmgren maintains that red-and-green blindness may occur separately ; 

 but, on the other hand, Hering and Stilling have maintained that they are always 

 associated, and Dr. George Berry, an eminent ophthalmologist, has assured me, from 

 his own observations, that such is the case. 



Of the various methods of testing colour vision, that suggested by Seebeck is 

 most commonly employed. The individual is mainly tested with regard to his 

 sense of creen and red. He is shown skeins of wool, one pale green, another pink 

 or purple, and a third bright red, and he is asked to select from a heap of coloured 

 wools, laid on a white cloth, the colours that appear to him to match those of the 

 several tests. We have arranged such test skeins for your inspection, and have 

 placed beneath each of them the colours which a red-green blind person usually 

 selects as having hues similar to those of the test. It is startling enough to find 

 brown, orange, green, and grey confused with bright red; pale red, orange, yellow, 

 and grey confused with green ; blue, violet, and green confused with pink ; but 



1 ' How do the Colour-blind Sec the different Colours ? ' Proc. Itrnj. Soe. Lo:id., 

 ISftl, vol. xxxi. p. ;WG. 



- Report on Colour Vision, lib. elf , plate i. No. 4. 



