Sir J. F. W. Herschel on Colowr-Blindness. 155 
which is to him absolutely insensible,”’ and that this red corresponds 
not to that colour which, under the name of carmine, offers to the 
normal-eyed the beau-ideal of redness, but what they term “ crim- 
son.’ Invisibility, as an element of colour, must notj 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. Ina 
cirele 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, 7. 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 are, 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 Jess 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, 
on an average, of one soldier in every fifty-five unable to distinguish 
a searlet coat from green grass would not issue in grave inconve- 
nience, and ere this have forced itself into prominence by pro- 
ducing mischief. Among the circle of my own personal acquaint- 
ance I have only known two (though, of course, I have heard of 
and been placed in correspondence with several) ; and a neighbour 
of mine, who takes great delight in horticulture, and has a superb 
collection of exotic flowers, informs me that among the multitude of 
persons who have seen and admired it, he does not recollect having 
ever met with one who appeared incapable of appretiating the variety 
