This is a very pleasing and instructive optical apparatus, and it may
be easily made by the young optician. Procure an oblong box, about two
feet long, twelve inches wide, and eight high. In one end of this a
tube must be fitted containing a lens, and be made to slide backwards
and forwards so as to suit the focus. Within the box should be a plain
mirror reclining backwards from the tube at an angle of forty-five
degrees. At the top of the box is a square of unpolished glass, upon
which from beneath the picture will be thrown, and may be seen by
raising the lid. To use the camera place the tube with the lens on it
opposite to the object, and having adjusted the focus, the image will
be thrown upon the ground-glass as above stated, where it may be easily
copied by a pencil or in colors.
The camera obscura used in a public exhibition is a large wooden box
stained black in the inside, and capable of containing from one to
eight persons. It contains a sliding piece, having a sloping mirror
and a double convex lens which may with the mirror be slid up or down
so as to accommodate the lens to near and distant objects. When the
rays proceeding from an object without fall upon the mirror they are
reflected upon the lens, and brought to fall on the bottom of the box,
or upon a table placed horizontally to receive them, which may be seen
by the spectator.