EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 141 



of any method of determining by a legitimate process the nature of the other 

 two sensations, although Young's reasons for adopting something like green and 

 violet appear to me worthy of attention. 



The only remaining subject to which I would call the attention of the 

 Society is the effect of coloured glasses on the colour-blind. Although they can- 

 not distinguish reds and greens from varieties of gray, the transparency of red 

 and green glasses for those kinds of light is very different. Hence, after finding 

 a case such as that in equation (4), in which a red and a green appear iden- 

 tical, on looking through a red glass they see the red clearly and the green 

 obscurely, while through a green glass the red appears dark and the green light. 



By furnishing Mr X. with a red and a green glass, which he could dis- 

 tinguish only by their shape, I enabled him to make judgments in previously 

 doubtful cases of colour with perfect certainty. I have since had a pair of 

 spectacles constructed with one eye-glass red and the other green. These Mr X. 

 intends to use for a length of time, and he hopes to acquire the habit of discri- 

 minating red from green tints by their different effects on his two eyes. Though 

 he can never acquire our sensation of red, he may then discern for himself what 

 things are red, and the mental process may become so familiar to him as to act 

 unconsciously like a new sense. 



In one experiment, after looking at a bright light, with a red glass over one 

 eye and a green over the other, the two tints in experiment (4) appeared to him 

 altered, so that the outer circle was lighter according to one eye, and the inner 

 according to the other. As far as I could ascertain, it appeared as if the eye 

 which had used the red glass saw the red circle brightest. This result, which 

 seems at variance with what might be expected, I have had no opportunity of 

 verifying. 



This paper is already longer than was originally intended. For further 

 information I would refer the reader to Newton's Opticks, Book i. Part n., to 

 Young's Lectures on Natural Philosophy, page 345, to Mr D. R Hay's works on 

 Colours, and to Professor Forbes on the "Classification of Colours" (Phil, Mag., 

 March, 1849). 



The most remarkable paper on the subject is that of M. Helmholtz, in the 

 Philosophical Magazine for 1852, in which he discusses the different theories of 

 primary colours, and describes his method of mixing the colours of the spectrum. 

 An examination of the results of M. Helmholtz with reference to the theory 



