mit to a COUNTRY PRACTITIONER the following
very simple test for the coincidence of the chemical and visual foci of an
achromatic lens:

Take a common hand-bill or other sheet of printed paper, and having
stretched it on a board, place it before the lens in an oblique position,
so that the plane of the board may make an angle with a vertical plane of
about thirty or forty degrees. Bring any line of type about the middle of
the sheet into the true visual focus, and take a copy of the sheet by
collodion or otherwise. Then, if the line of type focussed upon be
reproduced clearly and sharply on the plate, the lens is correct; but if
any other line be found sharper than the test one, the foci disagree; and
the amount of error will depend on the distance of the two lines of type
one from the other on the hand-bill.

J. A. MILES.

Fakenham, Norfolk.

_Improvement in Positives._--I have great pleasure in communicating to you
an improvement in the process of taking positives, which may not be
uninteresting to some of your readers, and which ensures by far the most
beautiful tints I have yet seen. I take three ounces of the hyposulphite of
soda, and dissolve it in one pint of distilled or rain water; and to this I
add about one or one and a half grains of pyrogallic acid, and seventy
grains {534} of chloride of silver; which must be squeezed up between the
finders facilitate its solution and separate the lumps, which, in their dry
state, are tough, and not easily pulverised. The whole is then to be set
aside for a week or two in a warm place. The solution, at first colourless,
becomes brown, and ultimately quite opaque; in this state it is fit for
use, and the longer kept the better it becomes. I generally use French
paper for this process, and, according to the time of immersion, obtain
fine sepia or black tints; the latter requiring long over-exposure to the
light, and proportionately long exposure to the action of the liquid; which
however will be found, particularly when old, to have a more rapid action
than most other setting liquids, and has the merit of always affording fine
tints, whatever the paper used. I imagine the pyrogallic acid to possess a
reducing influence on the salts of silver employed; but this effect is only
produced by its combination with the hyposulphite of soda and chloride of
silver. I may add, that in any case the pictures should be much overdone
before immersion, as the liquid exerts a rapid bleach