Mr Dalton on the Vision of Colours. 97 



medium, as Mr Delaval has proved, is to transmit more, and 

 consequently imbibe fewer of the rays of its own colour, than 

 of those of other colours. Reflecting upon these facts, I was 

 led to conjecture that one of the humours of my eye must be 

 a transparent, but coloured, medium, so constituted as to ab- 

 sorb red and green rays principally, because I obtain no pro- 

 per ideas of these in the solar spectrum ; and to transmit blue 

 and other colours more perfectly. What seemed to make 

 against this opinion, however, was, that I thought red bodies, 

 such as vermilion, should appear black to me, which was con- 

 trary to fact. How this difficulty was obviated will be under- 

 stood from what follows. 



Newton has sufficiently ascertained, that opaque bodies are 

 of a particular colour from their reflecting the rays of light of 

 that colour more copiously than those of the other colours ; the 

 unreflected rays being absorbed by the bodies. Adopting this 

 fact, we are insensibly led to conclude, that the more rays of 

 any one colour a body reflects, and the fewer of every other 

 colour, the more perfect will be the colour. This conclusion, 

 however, is certainly erroneous. Splendid coloured bodies re- 

 flect light of every colour copiously ; but that of their own 

 most so. Accordingly we find, that bodies of all colours, 

 when placed in homogeneal light of any colour, appear of that, 

 particular colour. Hence a body that is red may appear of 

 any other colour to an eye that does not transmit red, accord- 

 ing as those other colours are more copiously reflected from 

 the body, or transmitted through the humours of the eye. 



It appears, therefore, almost beyond a doubt, that one of the 

 humours of my eye, and of the eyes of my fellows, is a colour- 

 ed medium, probably some modification of blue. I suppose it 

 must be the vitreous humour; otherwise I apprehend it might 

 he discovered by inspection, which has not been done. It is 

 the province of physiologists to explain in what manner the 

 humours of the eye may be coloured, and to them I shall leave 

 it; and proceed to show that the hypothesis will explain the 

 facts stated in the conclusion of the second part. 



1. This needs no further illustration. 



Z. Pink is known to be a mixture of red and blue; that is, 

 these two colours are reflected in excess. Our eyes only trans- 



MEW SERIES, VOL. V. NO. I. JULY 1831. <: 



