MODIFIED BY COLOURED GLASS MEDIA, ETC. 
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red, orange and yellow glasses upon the photogenic papers and the Daguerreotype 
plate ; and this continuing action is probably due to the distinct photogenic action 
possessed by these rays, as I am able to prove by facts of a very positive nature. 
These two photogenic actions result from two different principles, nevertheless 
producing similar effects, as to the colour obtained, on the iodide, bromide or chloride 
of silver, whether it be found isolated, as is the case on the photogenic paper, or it 
be found in the presence of metallic silver, as happens upon the Daguerreotype plate ; 
but they produce quite an opposite effect upon the silver plate, whatever may be the 
colour previously given to the surface by these two radiations, endowing it with a 
property, the one of attracting, the other of repelling the mercurial vapours. We 
must take care not to confound these two results ; we can conceive two different 
actions giving the same colour to the iodide of silver, and we can also conceive that 
these two actions may be endowed with contrary properties as regards the fixation 
of mercurial vapour. 
The facts pointed out by M. Gaudin are the results of an action which does not 
belong to the Daguerreotype, since they are manifested without the aid of mercury; 
for we must not lose sight of the fact, that the production of the Daguerreotype 
image is due only to the affinity for mercury of the parts previously affected by the 
photogenic rays. It does not then follow from the production of an image without 
mercury, by crystallization or some peculiar arrangement of the molecules, that the 
red, orange, and yellow rays exert a continuing action analogous to that which deter- 
mines the fixation of mercurial vapour. 
The experiments of Sir J. Herschel, of Dr. Draper, of M. Lerebours, and of 
Messrs. Foucault and Fizeau, to prove the protective and destructive action of 
the red rays, were made with the prism. 
These philosophers have thus operated with the isolated rays in all their natural 
purity, and after them it would have been useless to seek to confirm or to contradict 
experiments so ably conducted and so conclusive. 
Sir J. Herschel, in a memoir published in the Philosophical Magazine for February 
1842, approves only of experiments made by means of the prism, as they are less 
subject to error from the foreign rays, which the coloured glasses never entirely 
exclude. This observation is perfectly just in theory, but in practice, in the par- 
ticular case of the photogenic power of different rays and of their different actions, 
it will be found that these phenomena can be studied with greater facility by using 
coloured glasses, and that the feeble quantity of foreign rays which they admit, far 
from interfering with the deductions of the experimenter, serve only to confirm and 
to render them more conclusive. We shall presently see that these foreign rays are 
completely neutralized in this class of experiments, and it would have been unfortu- 
nate not to have added these tests to those of the solar spectrum, since by the aid of 
coloured glasses I have been enabled, not only to confirm certain properties of the 
pure spectrum, but also to discover some others which had escaped my predecessors. 
