154 Royal Society :—_ 
leans to this opinion. I had satisfied myself, however, in the case 
of the late Mr. Troughton, that the extreme red—that pure and de- 
finite red which is seen in the solar spectrum only when the more 
luminous red is suppressed, and in which I cannot persuade myself 
that any yellow exists, was not invisible to him,—though of course not * 
seen as red; and on supplying Mr. Pole with a specimen of a glass, 
so compounded of a cobalt-blue and a red glass as to transmit posi- 
tively no vestige of any other ray, but that copiously, so that a 
eandle seen through it appears considerably luminous, and the 
window-bars against a cloudy sky are well seen if other light be kept 
from falling on the eye,—I am informed by him that he saw through 
it “gas, fire and other strong lights with perfect distinctness,” and 
that the colour so seen is a “very deep dark yellow.” Now it seems 
to me impossible to attribute this to any minute per-centage of yellow 
light of the same refrangibility, which this can be supposed to con- 
tain. The purity of its tint is extraordinary ; and its total intensity 
so small, that supposing it reduced to one-tenth of its illuminating 
power by the suppression of the whole of its primary red constituent, 
I cannot imagine that any gas-flame or fire-light would be visible 
through it, or any other luminous body but the sun, 
Still it remains a fact, however explained, that the red rays of the 
spectrum generally are to the colour-blind comparatively but feebly 
luminous. Mr. Pole speaks of red in more places than one as “a 
darkening power ;” and in the letter I have received from him in 
reply to my query as to the visibility of light through the red glass 
above mentioned, he insists strongly on its action as darkness. This, 
however, can only be understood of the effects of red powders in 
mixture, and not of red light; and as, to our eyes, an intense blue 
powder, such as prussian blue, has, besides its colorific effect, a vio- 
lent darkening one (owing to its feeble luminosity), so, to the colour- 
blind, red powders, when added to others, contribute but little light 
in proportion to the bulk they occupy in the mixture, and therefore 
exercise a darkening power by displacing others more luminous than 
themselves. I think it therefore very probable that red appears to 
the colour-blind as yellow-black does to the normal-eyed, or, in 
other words, that our higher reds are seen by them as we see that 
shade of brown which verges to yellow—that of the faded leaf of the 
tulip-tree for instance. Now it is worthy of remark, that it is very 
difficult for the normal-eyed to become satisfied that the browns are 
merely shades of orange and yellow. Brownness (such at least has 
always been my own impression) is almost as much a distinct sensation 
as greenness; so that I am not at all surprised at the expression in 
§ 22, that the “sensation of red as a dark yellow is certainly very 
distinct from full yellow,” or that a colour-blind person should, after 
long and careful investigation, arrive at the conclusion that red is 
not to him a distinct colour. I find all this completely applicable 
to my own perception of the colour brown. 
Mr. Pole (§ 11) appears to lay great stress on the fact,'that in a 
closed colour circle in which red, yellow, and blue are so arranged 
that each shall graduate into both the others, there occurs in the 
space where red and blue graduate into each other, “a hue of red 
