LXXXI
can produce only virtual erect images, which are nearer and
smaller than the object. 383.
Remark. By the convex lens a convergence of the rays, by the con
cave lens a divergence of the rays will be caused.
Optical instruments.
Besides burning-glasses and double convex lenses used for
reading we name the following:
§ 200. 1) Camera obscura (dark room), by which all ob
jects which can reach the hole of a dark shutter are depicted on
the oposite screen (invented by Porta 1650). By fixing a con
vex lens in the orifice and placing a white screen in its focus the
image becomes brighter and more definite. 384. This camera
obscura may be used in drawing by tracing with a pencil on a
thin paper laid upon the screen the outlines of the image. The
most important application of this camera obscura we see in
§ 201. 2) Photography (see No. 83), which is the art to
fix the images produced by the camera obscura on substances
sensitive to light. Different substances have been used for this, but
the problem was only solved in an efficient manner by Daguerre
in the year 1839. At first the photograph was taken on metal plates
which are not in use now, glass and paper having taken their
place. Also the chemical substances employed for this purpose
were infinitely changed and the beauty and perfection of the
present results we chiefly owe to the improvements of Talbot
and Archer. The method somewhat complicated for people
not acquainted with Chemistry is in short this. The camera
obscura of photographers consists of a wooden box, one part
of which is fixed, while the other one can be pushed in or out
like a drawer. In the tube fixed in front of the box are
contained one or two convex lenses, which can be moved
backwards and forwards by means of a cock-work until the
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