142 REPORT—1850. 
particularly of sulphur, as acting on silver in the same way as iodine ; and that 
caloric produced the same effect by oxidizing the metal, for from this cause 
proceeded in all these instances this extreme sensibility to light.” 
Mr. Henry Fox Talbot commenced his experiments in photography in 1834; 
and in 1839, about six months prior to the publication of Daguerre’s process, 
he published ‘Some Account of the Art of Photogenic Drawing, or the pro- 
cess by which natural objects may be made to delineate themselves without 
the aid of the artist’s pencil.’ Mr. Talbot, pursuing his inquiries, discovered 
his extremely sensitive process, the ‘‘ calotype,’’ which consists in exalting 
the sensibility of iodide of silver by the action of gallic acid. 
From the period of the announcement of the discovery of Daguerre in 1839, 
the inquiry into the phenomena connected with the chemical action of the 
solar radiations assumed a more inductive character ; and having no longer to 
record isolated discoveries made at far distant intervals, the historical arrange- 
ment will now be abandoned for a more philosophical examination of the 
subject. 
All the observations which had been made on the influence of the prismatic 
rays upon the salts of silver and other bodies, point to a very remarkable dif- 
ference in the action of the several rays; and this class of observations being 
more completely carried out by living philosophers, among whom Sir John 
Herschel demands the most distinguished notice, a still larger number of 
curious facts were elicited. 
It is the peculiar habit of our minds to endeavour to explain new phzeno- 
mena by received theories, and thus, I fear too often, to create imaginary re- 
semblances where no real analogies exist. In this way it has, I think, been too 
hastily decided that the varieties of action observed in the colorific rays of the 
spectrum in relation to chemical change are due to varying undulations and to 
the phenomena of wave interferences. ‘The amount of mathematical skill 
which has been brought to bear on the wave theory of light, has placed it in 
a most popular position; but, without for one moment attempting an objec- 
tion to any part of this theory as it explains luminous phenomena, it cannot 
be too strongly insisted on, for it is too often forgotten, that excepting Dr. 
Young’s experiment already quoted, no attempt has ever been made by any 
mathematician to associate the chemical agency of the solar rays with the 
theory of luminous undulations. Speculations there haye been, but all these 
have been ventured on without any attempt at analysis. This is particularly 
mentioned to show that the entire subject remains open for examination, and 
that this examination should be prosecuted without reference to any precon- 
ceived hypotheses. 
Sir John Herschel* remarks on “the high probability at least that the 
chemical energy is distributed throughout the spectrum in such a way as to 
be by no means a mere function of the refrangibility, but to stand in relation 
to other physical qualities, both of the ray, and of the analysing medium, and 
that relation by no means the same as that which determines the absorptive 
action of the latter on the colorific rays.” 
Without stopping to consider the question of the colours of the bands con- 
stituting the spectrum, it being perfectly indifferent to this subject whether 
we adopt the seven rays of unequal refrangibility of Sir Isaac Newton or the 
views arrived at by the refined researches of Sir David Brewster, which reduce 
the chromatic phenomena tothree,—Ishall at once pass tothe chemical agency 
exercised along different lines of the spectrum. It is however necessary that 
* On the Chemical Action of the Rays of the Solar Spectrum, &c. Philosophical Transac- 
tions, 1840, pt. 1. 
