Sir J. F. W. Herschel on Colour-Blindness. 149 
paper on the same subject*, and communicated at the request of the 
President and Council. | 
I consider this paper as in many respects an exceedingly valuable 
contribution to our knowledge of the curious subject of colour-blind- 
ness—Ist, because it is the only clear and consecutive account of 
that affection which has yet been given by a party affected, in pos- 
session of a knowledge of what has yet been said and written on it 
by others, and of the theories advanced to account for it, and who, 
from general education and habits of mind, is in a position to discuss 
his own case scientifically ; and 2ndly, for the reasons the author 
himself alleges why such a person is really more favourably ‘situated 
for describing the phenomena of colour-blindness, than any normal- 
eyed person can possibly be. It is obvious that on the very same 
principle that the latter considers himself entitled to refer all his per- 
ceptions of colour to three primary or elementary sensations— 
whether these three be red, blue, and yellow, as Mayer (followed in 
this respect by the generality of those who have written on colours) 
has done, or red, green, and violet, as suggested by Dr. Young, 
reasoning on Wollaston’s account of the appearance of the spectrum 
to his eyes—on the very same principle is a person in Mr. Pole’s 
condition, or one of any other description of abnormal colour-vision, 
quite equally entitled to be heard, when he declares that he refers his 
sensations of colour to two primary elements, whose combination in 
various proportions he recognizes, or thinks he recognizes, in all hues 
presented to him, and which, if he pleases to call yellow and blue, no 
one can gainsay him; though, whether these terms express to him 
the same sensations they suggest to us, or whether his sensation of 
light with absence of colour corresponds to our white, is a question 
whicl must for ever remain open (although I think it probable that 
such is really the case). All we are entitled to require on receiving 
such testimony is, that the party giving it should have undergone 
that sort of education of the sight and judgment, especially with 
reference to the prismatic decomposition of natural and artificial 
colours, for want of which the generality of persons whose vision is 
unimpeachably normal, appear to entertain very confused notions, 
and are quite incapable of discussing the subject of colour in a 
manner satisfactory to the photologist. 
It is as necessary to distinguish between our sensations of colour, 
and the qualities of the light producing them, as it is to distinguish 
between bitterness, sweetness, sourness, saltness, &c., and the che- 
mical constitution of the several bodies which we call bitter, sweet, 
&c. Whatever their views of prismatic analysis or composition 
might suggest to Wollaston and Young, I cannot persuade myself 
that either of them recognized the sensation of greenness as a con- 
stituent of the sensations they received in viewing chrome yellow, or 
the petal of a Marigold on the one hand, and ultramarine, or the 
blue Salvia on the other ; or that they could fail to recognize a certain 
redness in the colour of the violet, which Newton appears to have 
had in view when he regarded the spectrum as a sort of octave of 
colour, tracing in the repetition of redness in the extreme refrangible 
* Phil. Mag. vol. xiii. p, 282. 
