175 



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 and yellow 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-blind 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. 

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

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

 these. The colour of light, or the hue resulting from their combi- 

 nation, may be called green, white, or grey, at pleasure. It is shown 

 that this explanation of colour-blind vision will fully account for the 

 whole of the various symptoms above enumerated. Red and green, 

 for example, are both seen only as shades of yellow, and if the yellow 

 is of the same intensity in each, they will appear alike, and of course 

 be confounded with each other. 



The author then proceeds to consider how far his own case may 

 be regarded as a type of the defect in general. The varied and in- 

 congruous nature of the symptoms has given rise to a belief that 

 there are many varieties of colour-blindness, or at least many 

 different degrees of severity ; but after carefully examining the pub- 

 lished accounts, he has arrived at the conviction that the true di- 



