_Test for Lenses._--I beg to submit 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.