d film and
back again into the vessel, repeating this with the same portion of fluid
several times. Presently the fluid grows brownish, and at the same time
the whole picture gains the depth of shadow in its darker parts which we
desire. Again we place it under the running water. When it is well washed,
we plunge it into this bath of hyposulphite of soda, which removes all the
iodide of silver, leaving only the dark metal impregnating the film. After
it has remained there a few minutes, we take it out and wash it again as
before under the running stream of water. Then we dry it, and when it is
dry, pour varnish over it, dry that, and it is done. This is a
_negative_,--not a true picture, but a reversed picture, which puts
darkness for light and light for darkness. From this we can take true
pictures, or _positives_.

Let us now proceed to take one of these pictures. In a small room, lighted
by a few rays which filter through a yellow curtain, a youth has been
employed all the morning in developing the sensitive conscience of certain
sheets of paper, which came to him from the manufacturer already glazed by
having been floated upon the white of eggs and carefully dried, as
previously described. This "albumenized" paper the youth lays gently and
skilfully upon the surface of a solution of nitrate of silver. When it has
floated there a few minutes, he lifts it, lets it drain, and hangs it by
one corner to dry. This "sensitized" paper is served fresh every morning,
as it loses its delicacy by keeping.

We take a piece of this paper of the proper size, and lay it on the
varnished or pictured side of the negative, which is itself laid in a
wooden frame, like a picture-frame. Then we place a thick piece of cloth
on the paper. Then we lay a hinged wooden back on the cloth, and by means
of two brass springs press all close together,--the wooden back against
the cloth, the cloth against the paper, the paper against the negative. We
turn the frame over and see that the plain side of the glass negative is
clean. And now we step out upon the roof of the house into the bright
sunshine, and lay the frame, with the glass uppermost, in the full blaze
of light. For a very little while we can see the paper darkening through
the negative, but presently it clouds so much that its further changes
cannot be recognized. When we think it has darkened nearly enough, we turn
it over, open a part of the hinged back, turn down first a portion of th