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XVII. On different Properties of Solar Radiation producing or preventing a deposit of 
Mercury on Silver Plates coated with Iodine , or its compounds with Bromine or 
Chlorine, modified by Coloured Glass Media and the Vapours of the Atmosphere. 
By A. Claudet, Esq. Communicated by Sir David Brewster, F.R.S., fyc. 8$c. 
Received June 10, — Read June 17, 1847. 
From the commencement of photography it has been known that the red, orange, 
and yellow rays exert but a very feeble photogenic influence on the Daguerreotype 
plate. The experiments of several philosophers, especially those of Sir J. Herschel 
on photogenic papers, published in February 1840, prove that this action is more par- 
ticularly confined to the most refrangible part of the prismatic spectrum, commencing 
from the space found covered by the blue rays and extending to the extremity of the 
violet, and sometimes even beyond it. 
In 1839, Sir J. Herschel observed that the red rays exercised on several photo- 
genic papers an antagonistic action to the photogenic rays, modifying their effect. 
Contrary to this, in 1841, M. Ed. Becquerel presented to the Paris Academy of 
Sciences a memoir, in which he announced that the red, orange and yellow rays 
were endowed with the property of continuing the action commenced by the photo- 
genic rays; these latter he called exciting rays ; to the first he gave the name of 
continuing rays. 
M. Ed. Becquerel made his experiments on photogenic papers, and added that 
he had observed the same effects on the iodized silver plate. 
Dr. Draper of New York published in the Philosophical Magazine for November 
1842, some remarks on a class of rays which he supposed to exist in the light of the 
brilliant sun of Virginia, and which had the property, when separated, of entirely 
suspending the action of the diffused light from the sky ; these antagonistic rays ex- 
tended from the blue to the extremity of the red, and appeared to be almost as active 
in preventing the decomposition of the iodide of silver as the blue rays were in pro- 
ducing it. 
In January 1845 a memoir was read by me at the Society of Arts, London, in a 
part of which I recommended opticians to construct object-glasses in which they 
should particularly correct the chromatic aberration of the long photogenic space of 
the solar spectrum, even at the cost of the achromatism of the less refrangible rays. 
This, however, had been already indicated, without my being aware of it at the time, 
by Sir J. Herschel ; but I added that the greater separation of the visual and pho- 
togenic focus which might result from such a combination, according to the quality 
