potassium bought at a _proper market_ is certainly very
inconsiderable compared to the disappointment resulting from a false
economy.

H. W. DIAMOND.

Surrey County Asylum.

_Trial of Lenses._--When you want to try a lens, first be sure that the
slides of your camera are correctly constructed, which is easily done.
Place at any distance you please a sheet of paper printed in small type;
focus this on your ground glass with the assistance of a magnifying-glass;
now take the slide which carries your plate of glass, and if you have not a
piece of ground glass at hand, insert a plate which you would otherwise
excite in the bath after the application of collodion, but now _dull_ it by
touching it with putty. Observe whether you get an equally clear and
well-focussed picture on this; if you do, you may conclude there is no
fault in the construction of your camera.

Having ascertained this, take a chess-board, and place the pieces on the
row of squares which run {134} from corner to corner; focus the middle one,
whether it be king, queen, or knight, and take a picture; you will soon see
whether the one best in the visual focus is the best on the picture, or
whether the piece one or more squares in advance or behind it is clearer
than the one you had previously in focus. The chess-board must be set
square with the camera, so that each piece is farther off by one square. To
vary the experiment, you may if you please stick a piece of printed paper
on each piece, which a little gum or common bees'-wax will effect for you.

In taking portraits, if you are not an adept in obtaining a focus, cut a
slip of newspaper about four inches long, and one and a half wide, and turn
up one end so as it may be held between the lips, taking care that the rest
be presented quite flat to the camera; with the help of a magnifying-glass
set a correct focus to this, and afterwards draw in the tube carrying the
lenses about one-sixteenth of a turn of the screw of the rackwork. This
will give a medium focus to the head: observe, as the length of focus in
different lenses varies, the distance the tube is moved must be learned by
practice.

W. M. F.

_Is it dangerous to use the Ammonio-Nitrate of Silver?_--Some time ago I
made a few ounces of a solution of ammonio-nitrate of silver for printing
positives; this I have kept in a yellow coloured glass bottle with a ground
stopper.

I have, however, been much alarmed, and refrained from using it or