ns of them in the sequel of this work: but the present plate shews
one pictured in the contrary manner, viz. dark upon a white ground: or,
speaking in the language of photography, it is a _positive_ and not a
_negative_ image of it. The change is accomplished by simply repeating
the first process. For, that process, as above described, gives a white
image on a darkened sheet of paper: this sheet is then taken and washed
with a fixing liquid to destroy the sensibility of the paper and fix the
image on it.

This done, the paper is dried, and then it is laid upon a second sheet of
sensitive paper, being pressed into close contact with it, and placed in
the sunshine: this second process is evidently only a repetition of the
first. When, finished, the second paper is found to have received an
image of a contrary kind to the first; the ground being white, and the
image upon it dark.

[PLATE VIII. A SCENE IN A LIBRARY.]

PLATE VIII. A SCENE IN A LIBRARY.

PLATE VIII. A SCENE IN A LIBRARY.

Among the many novel ideas which the discovery of Photography has
suggested, is the following rather curious experiment or speculation. I
have never tried it, indeed, nor am I aware that any one else has either
tried or proposed it, yet I think it is one which, if properly managed,
must inevitably succeed.

When a ray of solar light is refracted by a prism and thrown upon a
screen, it forms there the very beautiful coloured band known by the name
of the solar spectrum.

Experimenters have found that if this spectrum is thrown upon a sheet of
sensitive paper, the violet end of it produces the principal effect: and,
what is truly remarkable, a similar effect is produced by certain
_invisible rays_ which lie beyond the violet, and beyond the limits of the
spectrum, and whose existence is only revealed to us by this action which
they exert.

Now, I would propose to separate these invisible rays from the rest, by
suffering them to pass into an adjoining apartment through an aperture in
a wall or screen of partition. This apartment would thus become filled
(we must not call it _illuminated)_ with invisible rays, which might be
scattered in all directions by a convex lens placed behind the aperture.
If there were a number of persons in the room, no one would see the other:
and yet nevertheless if a _camera_ were so placed as to point in the
direction in which any one were standing,