screws or clamps, similar to g. g.
fig. 29. Should the object to be copied be of unequal thickness, such
as a leaf, grass, &c., it will be necessary to place on the board,
first, a soft cushion, which may be made of a piece of fine flannel and
cotton wool. By this means the object is brought into closer contact
with the paper, which is of great consequence, and adds materially to
the clearness of the copy. The paper is now exposed to diffused
daylight, or, still better, to the direct rays of the sun, when that
part of the paper not covered by the object will become tinged with a
violet color, and if the paper be well prepared, it will in a short
time pass to a deep brown or bronze color. It must then be removed, as
no advantage will be obtained by keeping it longer exposed; on the
contrary, the delicate parts yet uncolored will become in some degree
affected. The photogenic paper will now show a more or less white and
distinct representation of the object. The apparatus figured at 29
consists of a wooden frame similar to a picture frame; a piece of plate
glass is fixed in front; and it is provided with a sliding cover of
wood, c., which is removed when the paper is ready to be exposed to the
action of the light. The back, d., which is furnished with a cushion,
as just described, is made to remove for the purpose of introducing the
object to be copied, and upon it the prepared paper; the back is then
replaced, and, by aid of the cross piece and screw, e., the whole is
brought into close contact with the glass.

The objects best delineated on these photographic papers, are lace,
feathers, dried plants, particularly the ferns, sea-weeds and the light
grasses, impressions of copper plate and wood engravings, particularly
if they have considerable contrast of light and shade--(these should be
placed with the face downwards, having been previously prepared as
hereafter directed)--paintings on glass, etchings, &c.

To fix the Drawings.--Mr. Talbot recommends that the drawings should be
dipped in salt and water, and in many instances this method will
succeed, but at times it is equally unsuccessful. Iodide of potassium,
or, as it is frequently called, hydriodate of potash, dissolved in
water, and very much diluted, (twenty-five grains to one ounce of
water,) is a more useful preparation to wash the drawings with; it must
be used very weak or it will not dissolve the unchanged muriate only,
as is intended but the black