22 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
Chemical Properties of the Red end of the Spectrum. 
60. August 2 7, 1839. — A highly concentrated spectrum having been formed on 
muriated paper, and kept for some time on the same spot, a coloured picture was 
formed, as in Art. 54. On examining this picture by dispersed daylight, I now for 
the first time noticed, that in all that part of the paper on which the full red of the 
spectrum had fallen, there was an appearance of whiteness, a sort of white prolonga- 
tion of, or appendage to, the dark photographic impression, contrasting itself with the 
general ground of the paper, which, under the influence of dispersed light from passing 
clouds, sky, & c., had acquired a very sensible discoloration. This discoloration had, 
therefore, as would appear, been counteracted by the red rays of the spectrum, which 
are, therefore, by no means to be regarded as inactive, but rather, at least in this in- 
stance, as exciting an action of an opposite nature to that of the blue, violet, and la- 
vender rays. To generalize this conclusion, and to assume a kind of positive and 
negative polarity in the spectrum, was the first and natural idea which presented itself 
on making the above experiment. Such a polarity had been suspected — asserted in- 
deed — either on a priori grounds, or on the strength of vague and loose reasonings 
founded on the general aspect of many natural phenomena. But almost the very first 
step in the further progress of the inquiry sufficed to prove the existence of something 
beyond a mere opposition of qualities. 
61. When paper already somewhat discoloured, by a short exposure to direct sun- 
shine, was used to receive the spectrum, this whiteness was no longer found to be 
produced ; but instead of such an appearance, the red tint, already noted as fringing 
the least refracted end of the photographic spectrum, was found to be not only much 
more lively, but to extend further ; and instead of terminating at or near the limit 
where the red and yellow blend into orange, as in the Table in article 54, to en- 
croach deeply on the space occupied by the red rays, and even, in some instances, 
entirely to cover that space. When, instead of exposing the paper in the first in- 
stance to direct sunshine, it was rendered considerably dark by exposing it in the 
violet rays of a prismatic spectrum, or in a sunbeam which had undergone the ab- 
sorptive action of a solution of am monio- sulphate of copper, the phenomenon in 
question was developed in great perfection ; the red rays of the condensed spectrum 
producing on such paper, not whiteness, but a full and fiery red, which occupied the 
whole space on which any visible red rays had fallen. 
62. In further illustration of this peculiar action, I therefore made the following 
experiment. On a piece of paper strongly darkened by exposure to the sun under 
a green glass which was ascertained to absorb every trace of the extreme red, I 
fastened a standard combination of glasses which transmitted only that red, being 
the identical combination described in my paper* above cited, and the same which 
I have always used in my experiments to insulate that ray in its utmost purity. The 
* Philosophical Transactions, 1820. 
