81 



son.'* Invisibility, as an element of colour, must not here be con- 

 founded with invisibility as light. It is certain that he sees the 

 crimson. It is not to him black, but (just what it ought to be on 

 the supposition that his vision is dichromic, and the union of his 

 colours produces white) a neutral, obscure grey ; grey being only an 

 abbreviated expression for feeble illumination by white light. In a 

 circle coloured with three elements graduating into each other, there 

 is no neutral point none, that is, where whiteness or greyness can 

 exist ; but when coloured with only two elements, such as yellow 

 and blue (positive yellow and blue, that is, whose union produces 

 white, not green), there are of necessity two neutral points which 

 would be both equally white, i. e. equally luminous, if the two ex- 

 tremities of each of the coloured arcs graduated off by similar de- 

 grees. But this not being the case with the yellow arc, one of its 

 ends to the colour-blind corresponding to a continuation of the red, 

 and so being deficient in illuminating power, the point of neutrality 

 will be that where a feebler yellow is balanced by a feebler blue, and 

 will therefore be less luminous, i. e. less white or more grey than 

 the other neutral spot. It is evident, from the general tenor of 

 Mr. Pole's expressions throughout this paper, that his ideas on 

 the subject of colour are gathered mainly from the study of pigments 

 and absorptive (i. e. negative) colours, and not from that of prismatic 

 (or positive) ones. In other words, his language is that of the 

 painter, as distinguished from the photologist ; the distinction con- 

 sisting in this that in the former colour is considered in its con- 

 trast with whiteness, in the other with blackness ; and thus it is 

 that black is considered by many painters as an element of colour, 

 as whiteness necessarily is by photologists. 



I may perhaps be allowed to add a few words as to the statistics 

 of this subject. Dr. Wilson gives it as the result of his inquiries, 

 that one person in every eighteen is colour-blind in some marked 

 degree, and that one in fifty-five confounds red with green. Were 

 the average anything like this, it seems inconceivable that the exist- 

 ence of the phenomenon of colour-blindness, or dichromy, should 

 not be one of vulgar notoriety, or that it should strike almost all 

 uneducated persons, when told of it, as something approaching to 

 absurdity. Nor can I think that in military operations (as, for in- 

 stance, in the placing of men as sentinels at outposts), the existence, 



VOL. x. G 



