Proceedings of Societies. 361 



Observations on Mr MaxwelVs Paper. By Dr G. Wilson. 1 



I greatly regret that indisposition will not allow me to attend the 

 meeting of the Royal Society this evening, especially after Mr Clerk 

 Maxwell has had the kindness to send me his MS. I should have 

 liked to express my admiration of his beautifully simple device for 

 testing, quantitatively as well as qualitatively, colour-vision, and of re- 

 ferring to the value of his results. Now that railway managers are 

 fully alive to the necessity of ascertaining the quality of colour- vision 

 of their servants, both for the sake of excluding the colour-blind and 

 of certifying the acuteness of visual perception of those who are to 

 handle and interpret railway-signals, the colour-top will prove of great 

 service in determining those points. Our regimental, naval, and hos- 

 pital surgeons also, but especially those on the recruiting service, will 

 have the opportunity (at least when the pressure of war is over) of 

 employing this instrument as a means of accumulating important data 

 in reference to the perception of colours. But on this I need not en- 

 large. 



In the cursory perusal which I have been able to give Mr C. Max- 

 well's paper, the points which have struck me most have been the follow- 

 ing, and, if agreeable to the Society, I should be glad if you would com- 

 municate them to it : — ■ 



1. It is satisfactory to find the author, while working independently, 

 and pursuing a mode of research peculiar to himself, reach the conclusion 

 concerning colour-blindness, that it is the habitual vision of two colours, 

 blue and yellow, whilst normal vision is the habitual perception of three, 

 blue, yellow, and red. Sir John Herschel, it now appears (viz. since the 

 publication of Dalton's Life) proposed, more than twenty years ago, to 

 distinguish colour-blindness as dichromic vision. I have urged the same 

 conclusion as making it vain to expect that more than one-third of 

 the phrenological organ of Colour (supposing such an organ to exist) 

 should be conspicuously wanting in the colour-blind, and as rendering it 

 hopeless to employ more than two-coloured signals, if those who are 

 colour-blind are allowed to act as signal-men. But though colour-blind- 

 ness may be conveniently referred to as identical with two-colour vision, 

 it seems questionable whether this is strictly accurate. The sensation of 

 red does not appear to be altogether absent from the colour-blind. On 

 the other hand, many of them distinguish red at times from blue and yel-. 

 low, as well as from green, and, so far as one may judge from their lan- 

 guage, their sensation of red is then the same as ours. Mr Maxwell's 

 experiments with the Cambridge students are not at variance with this 

 being the case. They show the great liability to mistake red which un- 

 questionably characterizes the colour-blind ; but the latter should never 

 see red if their eyes are devoid of the nervous apparatus essential to the 

 red sensation. I am inclined to think that, with very few exceptions, the 

 vision of every one is trichromic, though for practically useful purposes 

 it is only dichromic in the colour-blind. 



2. It seems to me exceedingly doubtful whether we sufficiently fully 

 define colour-blindness, even in reference to the utilitarian perception of 

 colours, by regarding it as equivalent to the non-perception of red. All 

 the records of colour-blind cases appear to show that the darker shades of 

 all colours are confounded with each other and with black, and the lighter 

 shades with each other and with white, in circumstances where the defect 

 of white light on the one hand, and the excess of it on the other, do not 



1 In a Letter to Professor Gregory. 



