AjA Scienfific Intelligence. [Jung, 
Gay-Lussac’s process succeeds tolerably well, but not unless a 
portion of the silver be precipitated. It is not, therefore, econo- 
mical; though perhaps it might be so conducted that the loss of 
silver is not equivalent to the saving of nitric acid. Berthollet 
affirms that the silver precipitated by copper always contains some 
copper. But the quantity must be extremely small.. This is the 
process always, I believe, followed by manufacturers when they 
wish to recover silver held in solution by nitrie acid. I consider the 
common method, hy means of muriate of soda and pearlashes as 
good as any, if properly conducted. 1 have performed it I dare say 
a hundred times, and the loss upon an ounce of silver never ex= 
ceeded a very few grains. The heat must not be too great nor too 
long continued, otherwise there is a risk of losing part, or even all, 
of the silver—T. 
IX. Queries respecting Plano-cylindric Lenses. 
(To Dr. Thomson.) 
SIR, 
I observed some time ago, in one of the numbers of the Reper- 
tory of Arts, an accouat of a new mode of constructing lenses for 
dioptrical instruments, stated to possess very great and peculiar 
advantages over the spherical ones at present in use. A French 
artist, it seems, has taken out a patent for constructing optical in- 
struments with those new lenses ; from which I am led to conclude 
that the discovery (if it be really such) is of some importance. But 
on the other hand, I am led to adopt a different opinion, from the 
circumstance of your not having mentioned it in your publication. 
However, as I am anxious to be informed on the subject, I beg 
ou will take some notice of it in a future number of the Annals. 
I should be glad to know whether those cylindrical lenses recom- 
mended by the Frenchman were ever thought of, or tried, before ; 
and if so, what are their peculiar properties; or if experiments 
have recently been made in this kingdom to ascertain their useful- 
ness, what has been the result. 
Again, as I have not been able to find any glass-grinder who 
would undertake to grind lenses of this new form (at least for a 
reasonable compensation), I should be greatly obliged to you, or 
some of your Correspondents acquainted with the art, to inform 
me of the best methods by which such lenses may be ground and 
polished; that, if possible, I might have some of them sufficient 
for such experiments as I might think proper to make, prepared 
under my owneye. This information I the more particularly re- 
quest, as 1 do not find any account of the mode of grinding lenses 
in Imison, or in the Encyclopedia. As the form of the lenses, 
however, is new, a new mode must be resorted to for preparing 
them. If the artists in Edinburgh, Birmingham, or London, have 
done any thing in this way, it would be only necessary to refer to 
them. But residing at such a distance, I cannot easily make the 
necessary inquiry, 
