ment
is often used in conjunction with the microscope, the mirror being
attached to the eye-piece and the tube of the microscope being placed
horizontally.

[Illustration: FIG. 1.]

About the beginning of the 19th century Dr Wollaston invented a simple
form of the camera lucida which gives bright and erect images. A
four-sided prism of glass is constructed having one angle of 90 deg.,
the opposite angle of 135 deg., and the two remaining angles each of
67-1/2 deg. This is represented in cross-section and in position in
fig. 2. When the pupil of the eye is held half over the edge of the
prism a, one sees the image of the object with one half of the pupil and
the paper with the other half. The image is formed by successive total
reflection at the surfaces b c and a b. In the first place an inverted
image (first image) is formed in the face b c, and then an image of this
image is formed in a b, and it is the outline of this second image seen
projected on the paper that is traced by the pencil. It is desirable for
two reasons that the image should lie in the plane of the paper, and
this can be secured by placing a suitable lens between the object and
the prism. If the image does not lie in the plane of the paper, it is
impossible to see it and the pencil-point clearly at the same time.
Moreover, any slight movement of the head will cause the image to appear
to move relatively to the paper, and will render it difficult to obtain
an accurate drawing.

[Illustration: FIG. 2.]

Before the application of photography, the camera lucida was of
considerable importance to draughtsmen. The advantages claimed for it
were its cheapness, smallness and portability; that there was no
appreciable distortion, and that its field was much larger than that of
the camera obscura. It was used largely for copying, for reducing or for
enlarging existing drawings. It will readily be understood, for example,
that a copy will be half-size if the distance of the object from the
instrument is double the distance of the instrument from the copy.
(C. J. J.)

CAMERA OBSCURA, an optical apparatus consisting of a darkened chamber
(for which its name is the Latin rendering) at the top of which is
placed a box or lantern containing a convex lens and sloping mirror, or
a prism combining the lens and mirror. If we hold a common reading lens
(a magnifying lens) in front of a lamp or some other bright object and
at some distance from it, and if