Sir J. F. W. Herschel on Colour-Blindness. 153 
set upon the line of junction a sheet of glass inclined to the plane of 
the papers upwards towards the eye, so as to allow the blue to be 
seen by transmitted light, while the yellow reflected from the glass 
was at the same time received into the eye. By varying the inclina- 
tion of the glass, the yellow reflexion could be made more or less 
vivid, so as either to be nearly imperceptible or quite to Azll the 
blue of the paper. But at no stage of its intensity, gradually in- 
creased from one to the other extreme, was the slightest tendency to 
greenness produced. The colour passed from blue to yellow, not 
through green, but through a pale uncertain purplish tint, not easy 
to describe, but as remote from green as could be well imagined. 
Of course in all such experiments one eye only must be used. 
Stereoscopic superposition of colour, which at first sight would ap- 
pear readily available, does not satisfy the requisite conditions, and 
yields no definite results. 
The conclusions from these facts may be summed up as follows :— 
1st. That in no case can the sensation of green be produced by the 
joint action on the eye of two lights, in neither of which, separately, 
prismatic green exists; 2ndly. That the joint action of two lights, 
separately producing the most lively sensations of blue and yellow, 
does not give rise to that of green, even when one of them contains 
in its composition the totality of green light in the spectrum; 
and, 3rdly. That all our liveliest sensations of yellow are produced by 
the joint action of rays, of which those separately exciting the ideas 
of red and green form a large majority ; and that a decided yellow 
impression is produced by the union of these only. 
From these premises it would seem the easiest possible step to 
conclude the non-existence of yellow as a primary colour. But this 
conclusion I am unable to admit in the face of the facts,—1st, that 
a yellow ray, incapable of prismatic analysis into green and red, may 
be shown to exist, both in the spectrum and in flames in which soda 
is present; and 2ndly, that neither red nor green, as sensations, are 
in the remotest degree suggested by that yellow in its action on the 
eye. Whether under these circumstances the vision of normal-eyed 
persons should be termed trichromic or tetrachromic, seems an open 
question. 
That Mr. Pole’s vision is dichromic, however, there can be no 
doubt. If I could ever have entertained any as to the correctness 
of the views I have embodied of the subject in that epithet, after 
reading all I haye been able to meet with respecting it, this paper 
would have dispelled it. That he sees blue as we do, there is no 
ground for doubting; and I think it extremely likely that his sensa- 
tion of whiteness is the same as ours. Whether his sensation of 
yellow corresponds to ours of yellow, or of green, it is impossible to 
decide, though the former seems to me most likely. 
One of the most remarkable of the features of this case, and in- 
deed of all similar ones, is the feebleness of the efficacy of the red 
rays of the spectrum in point of illuminating power, which certainly 
very strongly suggests an explanation drawn from the theory of three 
primary coloured species of light, to one of which the colour-blind 
may be supposed absolutely insensible. Mr, Pole himself evidently 
