THE
CAMERA LUCIDA
“The farther the prism is removed
from the paper, that is, the longer the stem is drawn out, the larger
the objects will be represented in the drawing and accordingly, the less
extensive the view. The nearer the prism is to the paper, the smaller
will be the objects, and the more extensive the view comprised on the
same piece of paper.
In copying drawings, the copy will
be larger or smaller than the original, according as the prism is more
or less distant from the paper than it is from the drawing to be
copied. Thus, if the drawing be two feet from the prism, and the paper
only one foot, the copy will be half the size of the original. If the
drawing be at one foot, and the paper three feet distant, the copy will
be three times as large as the original; and so for all other
distances.”
DIRECTIONS
FOR USING THE CAMERA LUCIDA
“The Instrument being fixed by the
screw and clamp to the table and paper on which the drawing is to be
made, its stem should be inclined so as to bring the prism nearly over
the centre of the paper, and the pin, on which the prism turns, placed
truly horizontal.
The Instrument, as represented in
the figures, may be used either with the small round glass, turned up in
front, Fig 1, or with the larger glass, turned up level underneath the
Instrument, Figure 2, (seen from above.) But those who are
short-sighted can only use the former, and persons that are long-sighted
must use the latter.
The prism is next to be turned
upon its pin, till the transparent rectangular face be placed opposite
to the objects to be delineated, when the upper black surface of the
eye-piece (Figure 2) will be on the top of the Instrument; and through
the aperture in this, the artist is to look perpendicularly downwards at
his paper.
The black eye-piece is moveable,
and in ordinary circumstances, is to be in such a position, that the
edge of the small transparent part at the back of the prism shall
intercept about half the eye-hole. The artist then, looking through the
eye-hole directly downwards at his paper, should see the objects he
wishes to draw, apparently distributed over the paper. For, since his
eye is larger than the eye-hole, he sees through both halves of the hole
at the same time, without moving his head. He sees the paper through
the nearer half, and sees the objects at the same time through the
farther half, apparently in the same direction, by means of reflection
through the prism.”
THE POSITION OF THE EYE-HOLE
“The position of the eye-hole is
the circumstance, above all others, necessary to be attended to in
adjusting the Camera Lucida for use; for, on the due position of this
hole depends the possibility of seeing both the pencil and the objects
distinctly at the same time.
If the eye-hole be moved, so that
nearly the whole of its aperture be over the paper, and a pencil and
paper will be very faint. But there will always be an intermediate
position (varying according as the objects or the paper happen to be
most illuminated) in which both will be sufficiently visible for the
purpose of delineation, though not quite so clear as to the naked eye.
This intermediate position is easily found, with a little practice.
If objects can be seen distinctly
on the upper part of the paper, but not upon the lower, the Instrument
requires to be turned upon its pin, so that the transparent face may be
inclined rather downwards, and the contrary for seeing the upper part of
the view.
Many persons, upon first
attempting to use this Instrument, occasionally lose sight of their
object or their pencil, merely by means of a little motion of their
head, backwards and forwards, of which they are not aware, in breathing;
but a very little practice soon obviates this difficulty.”