558 
ARTICLE XXIII. 
Considerations relative to the Chemical Action of Light. 
By M. Arago. 
[From the Comptes Rendus, No. 17, for Feb. 18, 1843.] 
. A LETTER from M. Edmond Becquerel gave rise to a verbal 
communication from M. Arago, which we shall give as faithfully 
as possible. 
A short time after the vote of the legislature which awarded 
a national recompense to MM. Daguerre and Niépce, very er- 
roneous opinions, according to my idea, were manifested by a 
small portion of the public, which however imposed on me the 
duty of showing that the new discovery should not be considered 
merely in an artistical point of view, but that it would enrich 
physics with very valuable means of investigation. Such was 
the intention of a Note which appeared in the Comptes Rendus of 
the sitting of the 19th August, 1839. It was worded as follows: 
“ Here is an application of which the Daguerreotype will be 
susceptible, and which appears to me very worthy of interest. 
“ Observation has shown that the solar spectrum is not con- 
tinuous, that there exist transversal solutions of continuity, lines 
entirely black. Are there similar solutions of continuity in the 
obscure rays which seem to produce the photogenic effects ? 
“If there are any, do they correspond with the black lines of 
the luminous spectrum ? 
“Since several of the transversal lines of the spectrum are 
visible to the naked eye, or when they are painted on the retina 
without any amplification, the problem which I have just laid 
down will easily be resolved.” 
This very easy solution of the problem which I had proposed 
I was not able in 1839 to search for experimentally myself, the 
old camera obscura of the observatory being at that time destined 
to another purpose, and the new one was not yet constructed. 
However, I must suppose that my appeal was attended to. I 
have learnt in fact that the Royal Society received a memoir the 
20th February, 1840, of Sir John Herschel’s, where the ques- 
tion is glanced at, and every one here remembers that M. Ed- 
