in
the camera is to place it between a piece of plate glass and some other
material with a flat surface, as a piece of smooth slate or an iron
plate, which latter, if made warm, renders the paper more sensitive,
and consequently the picture is obtained more rapidly.

Time of Exposure.--With regard to the time which should be allowed for
the paper to remain in the camera, no direct rules can be laid down;
this will depend altogether upon the nature of the object to be copied,
and the light which prevails. All that can be said is, that the time
necessary for forming a good picture varies from thirty seconds to five
minutes, and it will be naturally the first object of the operator to
gain by experience this important knowledge.

Bringing Out the Picture.--The paper when taken from the camera, which
should be done so as to exclude every ray of light--and here the dark
slide of the camera plate holder becomes of great use--bears no
resemblance to the picture which in reality is formed. The impression
is latent and invisible, and its existence would not be suspected by
any one not acquainted with the process by previous experiment. The
method of bringing out the image is very simple. It consists in
washing the paper with the gallo-nitrate of silver, prepared in the way
already described, and then warming it gently, being careful at the
same time not to let any portion become perfectly dry. In a few
seconds the part of the paper upon which the light has acted will begin
to darken, and finally grow entirely black, while the other parts
retain their original color. Even a weak impression may be brought out
by again washing the paper in the gallo-nitrate, and once more gently
warming it. When the paper is quite black, as is generally the case,
it is a highly curious and beautiful phenomenon to witness the
commencement of the picture, first tracing out the stronger outlines,
and then gradually filling up all the numerous and complicated details.
The artist should watch the picture as it developes itself, and when in
his judgment it has attained the greatest degree of strength and
clearness, he shall stop further proceedings by washing it with the
fixing liquid. Here again the mixed solution need not be used, but the
picture simply brushed over with the gallic acid.

The Fixing Process.--In order to fix the picture thus obtained, first
dip it into water; then partly dry it with bibulous paper, and wash it
with a solutio