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chromic affection is much move common than is generally sup- 

 posed. He points out reasons why the descriptions given by the 

 colour-blind of their sensations may often be imperfectly expressed 

 and easily misunderstood, alludes to the difficulty of explaining the 

 symptoms by any other hypothesis, or even of classifying them in 

 any consistent way ; and considers the fact exemplified in his own 

 case, that dichromic vision will explain all the phenomena, as 

 strongly corroborative of the uniformity insisted on. 



From the results of his investigations, the author draws a few in- 

 ferences in regard to the theory of the primary colours, although ad- 

 mitting his incompetence to deal fully with this part of the subject. 

 He considers that, from the extreme simplicity of the phenomena of 

 colours as seen by the colour-blind, their experience may serve as 

 a stepping- stone to the more complex problems of normal vision. 

 Their light is divisible into two colours, blue and yellow ; and since 

 these must be undoubtedly primaries to the colour-blind, it is rea- 

 sonable to infer they should also be primaries in the normal system. 

 The dichromic eye further becomes of use as an analyser of colours, 

 and can detect the presence of blue or yellow in compounds whose 

 elements may be inseparable to normal eyes. Thus it finds that in 

 orange there is much yellow, and in violet much blue, and therefore 

 these cannot be simple colours. Red, producing no impression on 

 the colour-blind eye, may be assumed to be a simple colour, and 

 may therefore be put down as the third primary, so that the phe- 

 nomena of colour-blindness would appear to confirm the ordinary 

 theory, or at least are more consistent with it than with any other. 

 The fact of carmine-red presenting to the colour-blind a decided 

 sensation of yellow, affords a confirmation of Sir David Brewster's 

 theory of the triple spectrum, according to which this result ought 

 to be expected. 



The principal symptom of colour-blindness being the mistaking of 

 red for green, and vice versa, it has been thought that the use of 

 these colours for railway and ship signals becomes dangerous where 

 colour-blind persons may have to observe them. The author points 

 out that this danger may be obviated by very simple means. Red 

 and green are not confounded with each other generally, but only 

 such hues of them as lie in both cases on the yellow side of the 

 neutral ; and therefore if the green be made a blue green at the same 



