we hold a sheet of paper vertically at
a suitable distance behind the lens, we see depicted on the paper an
image of the lamp. This image is inverted and perverted. If now we place
a plane mirror (e.g. a lady's hand glass) behind the lens and inclined
at an angle of 45 deg. to the horizon so as to reflect the rays of light
vertically downwards, we can produce on a horizontal sheet of paper an
unperverted image of the bright object (fig. 1), i.e. the image has the
same appearance as the object and is not perverted as when the
reflection of a printed page is viewed in a mirror. This is the
principle of the camera obscura, which was extensively used in
sketching from nature before the introduction of photography, although
it is now scarcely to be seen except as an interesting side-show at
places of popular resort. The image formed on the paper may be traced
out by a pencil, and it will be noticed that in this case the image is
real--not virtual as in the case of the camera lucida. Generally the
mirror and lens are combined into a single piece of worked glass
represented in section in fig. 2. Rays from external objects are first
refracted at the convex surface _a b_, then totally reflected at the
plane surface _a c_, and finally refracted at the concave surface _b c_
(fig. 2) so as to form an image on the sheet of paper _d e._The curved
surfaces take the place of the lens in fig. 1, and the plane surface
performs the function of the mirror. The prism _a b c_ is fixed at the
top of a small tent furnished with opaque curtains so as to prevent the
diffused daylight from overpowering the image on the paper, and in the
darkened tent the images of external objects are seen very distinctly.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

Quite recently, the camera obscura has come into use with submarine
vessels, the _periscope_ being simply a camera obscura under a new name.
(C. J. J.)

_History_.--The invention of this instrument has generally been
ascribed, as in the ninth edition of this work, to the famous Neapolitan
savant of the 16th century, Giovanni Battista della Porta, but as a
matter of fact the principle of the simple camera obscura, or darkened
chamber with a small aperture in a window or shutter, was well known and
in practical use for observing eclipses long before his time. He was
anticipated in the improvements he claimed to have made in it, and all
he seems really to have done was to popularize it. T