284 Royal Society : — 



that his vision is perfectly dichromic, and shows the appHcability to 

 it of the definition of this kind of vision given hy Sir John Herschel, 

 which he beheves has never hitherto been followed out so completely 

 as is necessary to explain the phenomena observed. 



Blue and yellow he sees perfectly well, and has no reason to doubt 

 that his sensations of these two colours are the same as those of the 

 normal-eyed. The third primary, red, is the one in regard to which 

 his vision is defective, but the study of the sensations produced, by 

 this colour has been involved in some difficulty. Carmine, the arti- 

 ficial representative of what is usually considered pure red, presents 

 to the author's eye a very positive sensation, which he long supposed 

 to be a distinct colour ; but on examining it more closely, he found 

 it to be merely a dark shade of yellow, as he could match carmine 

 red perfectly with a mixture of yellow and black. There is, how- 

 ever, a variety of red, namely crimson, which is perfectly invisible, as 

 a colour, to his eyes, giving only a sensation of darkness ; and the 

 whole of the hues of red and orange between this and yellow present 

 only different shades of the latter colour ; the red element appearing 

 to act, not as a coloui'ing agent, but simply as a darkening power. 

 The author has endeavoured to find the place of this, to him, neutral 

 or invisible hue of red on the spectrum, and believes that if it exists 

 there at all, it must be situated at one or both of the extreme ends, 

 a position which would appear to distinguish it as possessing some 

 peculiar property, and he offers a conjecture that this, and not car- 

 mine red, may perhaps be the true primary colour. 



The hues of violet, lying between blue and crimson, appear, on a 

 similar principle, only shades of blue, the red darkening the blue in 

 the same manner as the yellow. 



In passing on to the green division of the colour circle, lying 

 between the blue and yellow, the author calls attention to the appa- 

 rent anomaly, that though colour-blind persons see blue and yellow 

 perfectly well, their combination, green, should be so great a stum- 

 bling-block. This fact appears to have perplexed everybody who 

 has treated on the subject ; the author imagined he was the first to 

 discover the explanation, but he found he had been anticipated by 

 Sir John Herschel, who says in his letter to Dalton, " the equili- 

 brium of blue &\\A.ye\\.ov/ produces your white," i.e. the white of the 

 colour-blind is not white at all, but green. And this is consistent 

 with theory ; for if normal white is a combination of three elements, 

 the invisibility of one of these elements to the colour-olind should 

 naturally have the effect of changing the appearance of their com- 

 pound. Since, therefore, green is only a colour to the normal-eyed 

 as it is contrasted with white light, it becomes no colour at all to the 

 colour-blind. The author proves this by showing that a certain hue 

 of green exactly matches, to his eyes, a neutral grey ; that all greens 

 on the yellow side of this appear only shades of yellow, all on the 

 blue side, only shades of blue. 



Thus the dichromic explanation of the author's vision is complete. 

 lie has only two sensations of colour, properly so called, namely 

 blue and yellow, all other hues in nature being reduced to shades of 



