EEVIEWS — EESEABCHES 02C COLOcB-ELISDSESS. 159 



that colour-blindness consists essentially in an absolute insensibility 

 to one of the three tints assumed for primaries, or as Mr. Maxwell 

 expresses it, " in the absence of a determinate sensation, depending 

 perhaps upon some undiscovered structure or organic arrangement, 

 which forms one-third of the apparatus by which we receive sensation of 

 colour," a conclusion from which a subsequent remark of Dr. "Wilson 

 abstracts all the force, for he says ; " It is only in fully-developed 

 colour-blindness that vision is decidedly dichromic, and even then it 

 is not absolutely so, at least so far as my experience goes." In other 

 words, out of the infinite variety of colour-blindness there is one par- 

 ticular and limited class which may be described as dichromic. Such 

 an explanation of the phenomena in general is moreover inconsistent 

 with the fact of chromic myopia pointed out by Dr. "Wilson, which ex- 

 presses that the colour-blind can appreciate colours, when close, of which 

 they lose the distinction at a certain distance ; and still more with the 

 fact that even the normal eye becomes colour-blind to the shades of 

 certain colours by sufficient diminution of intensity; in short this ex- 

 planation exalts into a difference of kind, what our author himself more 

 than once strongly represents as being only a difference of degree. 



With that felicity of practical application which has always dis- 

 tinguished Dr. Wilson, he has devoted a considerable portion to- 

 wards the close of this work to a consideration of the dangers in- 

 volved in the possible employment of colour-blind persons on Bail- 

 ways or Vessels, and of the liabilities of mistake to which the present 

 system of signalling by coloured lights and flags is open ; we regret 

 that our space will not permit us to follow him through those in- 

 teresting and useful chapters, and shall content ourselves with 

 observing that, on the whole, Dr. Wilson recommends calling in the 

 aid oiform to that of colour, both by flags or signal-vanes of different 

 shapes for day, and by using differently-arranged combinations of 

 several lamps for night instead of a single one ; also that of all colour- 

 ed lights, the best practically would be, white for safety ; red for cau- 

 tion ; and for danger by day, sly-blue ; by night, yelloiv. This subject 

 strongly claims the attention of our Bail way authorities, although on 

 this side the Atlantic, accidents are sufficiently numerous from other 

 causes, to render this particular one of comparatively small moment. 



We lay down the volume with hearty thanks to Dr. Wilson both 



for his own experiments and researches in this obscure subject, and 



for having embodied all that is yet known about it in a clear and 



concise resume which will serve as a standard of reference hereafter 



to the scientific investigator. 



J. B. C. 



