2 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
salts in Brewster’s Edinburgh Philosophical Magazine*) of readily dissolving the 
chloride and other combinations of silver insoluble in the generality of menstrua. It 
has since appeared that this is the solvent used by M. Daguerre ; and I may add 
that it is the only one that I have found at once easy of management, and perfectly 
to be relied on under all circumstances, though there are many which occasionally, 
or with great attention to nicety of application, succeed well. 
4. 2ndly. Observation of the feeble susceptibility of the pure chloride of silver to 
the action of light, and attempts to attain a greater degree of susceptibility, attempts 
attended then with very limited and partial success, far surpassed by the curious and 
powerful processes of Mr. Talbot, not then divulged, as well as by those which I have 
myself since fallen upon. 
5. 3rdly. The application of photography to the copying of drawings and engravings, 
and to the fixation of optical images. Of course it will be understood that I have no 
intention here of interfering with Mr. Talbot’s just and long-antecedent claims in this 
or in any other points ; but having bestowed much attention both then and since on 
these processes, so as to produce results far from unsatisfactory, it will be necessary 
to say a few words in justification of the time and pains I have devoted to this branch 
of the inquiry. 
6. A susceptibility to the action of light, and a power of destroying at any given 
moment all susceptibility to further action in the ground of a picture, being granted, 
it obviously depends only on the degree of that susceptibility, and on the precision 
with which the rays of light can be concentered on or confined to their intended 
points of action, bow far the photographic impression obtained, whether from the ca- 
mera or the original drawing or engraving to be copied, shall satisfy the wishes of 
the artist. As regards the camera ; that which I employed was neither periscopic 
nor achromatic, being in fact no other than the aplanatic crown-glass lens computed 
and described in paragraph 13 of my paper on the aberrations of lenses'f-. This com- 
bination, though admirable for its original purpose as a burning-glass, is in fact one 
of the worst possible for a photographic camera, in which the three qualities of a flat 
field, a sharp focus at great inclinations of the visual ray, and a perfect achromaticity, 
are indispensable. The latter quality, indeed, (as will be apparent enough from the 
properties about to be described,) is even more necessary for the photographic than 
for the ordinary use of the camera. In consequence, my first camera pictures were 
necessarily extremely defective. Nor was I at all anxious on this head, knowing that 
the other essentials of the photographic problem being secured, the acquisition of a 
perfect camera would ensure perfection in that respect. 
7. A much more important line of inquiry, as far as applications are concerned, 
appeared to be the exact reproduction of indefinitely multiplied fac-similes of an ori- 
ginal photograph once obtained, by which alone the publication of originals could be 
accomplished. And this seemed the more deserving of attention, as it was under- 
* Philosophical Transactions, 1821. f 1819. vol. i. and 1820. vol. ii. 
