of
iron for five minutes, and wash as before. If preferred, the iron
solution may be washed over the prints, or they may be immersed in it,
but floating seems preferable. After the second washing, wash the prints
over with a strong solution of pyrogallic acid, when the print will
develop black, and the ground, if the washings were sufficient, will
remain white. A final washing completes the process.

If a solution of yellow prussiate of potash be used in place of the pyro
solution, a blue print is obtained. Bichromate prints can be made on
albumenized paper by floating it on the solution, and by using a
saturated solution of protosulphate of iron and a saturated solution of
gallic acid. Very fine prints can be so produced nearly equal to silver
prints, and at somewhat less cost, but with a little or no saving of time
or labor.

_Chief Proof Solution_.--If old oxalate developer be exposed in a shallow
vessel in a warm place, a deposit of light green crystals will be formed,
composed of an impure oxalate of iron. If these crystals be dissolved in
water, and paper washed with a strong solution, when dry it may be
exposed in the printing-frame, giving full time. The image is very faint,
but on washing in or floating on a moderately strong solution of red
prussiate of potash for a minute or less, a blue positive is produced,
which is washed in water as usual to fix it. The unused developer
produces the best crystals for the purpose, and the pure ammonio-oxalate
is vastly better than either.

All of the above operations, except the printing, should be carried on in
the dark room, or by lamp or gas light only. The solutions and the paper
should also be kept in the dark, and prepared as short a time as possible
before use.

II. COMPOUND NEGATIVES.

In photographing with the microscope, it frequently occurs that the
operator, instead of devoting a negative to each of two or more similar
objects for comparison, printing both upon the same print, prefers to
have the whole series upon one negative, and taking from this a single
print. There is often room for two or more images upon the same plate. If
the center of the plate is devoted to one, obviously no more can be
accommodated on it, but by placing one at each end, or one on each
quarter of the plate, both economy of plates and convenience of printing
are secured. The end may be readily accomplished by matting the plate as
a negative is matted in printing.

Suppose it