APPENDIX. 513 



colour of a precipitate. He examined the sight of his brother 

 professors and of his pupils, and had drawn up before him the 

 police, and the attendants at a lunatic asylum, and whole com- 

 panies of soldiers, infantry, artillery, and hussars. Thus he was 

 able to determine, with some accuracy, the proportion of the 

 colour-blind, about two per cent, being found defective as 

 Dalton was, that is, mistaking greens, reds, and browns, and 

 sometimes calling red black ; while about five per cent, were 

 found subject to this peculiarity in a minor degree. Dr. 

 Wilson's mode of experimenting was usually with coloured 

 diagrams and Berlin wools, but he also employed the prismatic 

 spectrum itself. Thus he was able to show the small perception 

 of red, which is the principal symptom of colour-blindness ; and 

 that many of the most curious mistakes, such, for instance, as 

 confounding light red purple with dark blue, arise from the red 

 rays being scarcely luminous to such patients. I believe he 

 was the first to point out that there is often a shortsightedness 

 in regard to colour when there is none in respect to form ; and 

 that to many patients red is more visible by artificial light, so 

 that, while unable to distinguish by their colour the red flowers 

 from the green leaves of the geranium by day, they enjoy the 

 chromatic contrast as they walk through the conservatory by 

 gas-light. On this fact Dr. Wilson founded the most practical 

 suggestion for the alleviation of this defect, namely, the substi- 

 tution of artificial light for day-light in the examination of 

 colours, and the employment, for a similar purpose, of glasses 

 coloured slightly orange or yellow. He was not content with 

 ascertaining the symptoms, but desired also to discover the 

 cause of colour-blindness ; hence he was led to examine the 

 theories which had been previously propounded, and to investi- 

 gate the necessaiy chromatic effect of the yellow spot on the 

 retina, the colour of the choroid coat, and of the tapetum lucidum 

 in animals, and the vision of Albinoes. These inquiries did not 

 lead to an explanation of the matter, but they were interesting 

 in themselves, and gave rise to two special papers, 1 which he 



i On the Extent to which the received Theory of Vision requires us to regard the Eye 

 as a Camera Obscura. ' Trans. E.S.E.' vol. xxi. part ii. 



On the Transmission of the Actinic rays of light through the Eye, and their relation 

 to the Yellow Spot of the Retina.' Trans. R.S.E.' 



2 K 



