THE CAMERA OBSCURA AND CAMERA LUCIDA.
THE construction of the camera obscura is founded on the fact that the
rays of light, when collected into a point either by being passed
through a small hole or a converging lens, form an image of the objects
from which they proceed at the point of meeting. This may be readily
tried by piercing the shutter of a room with a small hole, and holding a
piece of paper within a short distance of it. It will be noticed that
the smaller the hole the more distant will be the image formed. The
first person who observed this fact was John Baptist Porta, an Italian
philosopher who lived in the latter part of the seventeenth century. He
noticed that when a screen was placed opposite a small hole in the
shutter of his room, the objects outside were depicted on it in a
reversed position with moderate distinctness; but that when a biconvex
lens was placed over the hole, the picture was rendered much more
distinct. This was the first attempt at the formation of the camera
obscura, an instrument that has since bestowed such incalculable
benefits on humanity.
[Illustration:
FIG. 70.—The Camera Obscura.
]
The shape of the images so formed is independent of the shape of the
opening, which, as long as it is sufficiently small, may be square,
oval, or triangular. This may be easily seen when the sun shines through
the intervals between the leaves of a shady avenue or bower of trees.
The image of the sun as a circular patch of light is seen scattered over
the surface of the ground, although the accidental intervals formed by
the leaves above were of a thousand different shapes. These images at
the time of an eclipse of the sun are very surprising, taking, as they
do, the form of a crescent, more or less large according to the
magnitude of the eclipse.
This property possessed by the rays of light, of depicting on a screen
the forms and colours of the objects from which they proceed when passed
through a small aperture or a lens, is taken advantage of in most places
famous for their natural scenery. The apparatus employed for this
purpose is comparatively simple, consisting merely of a dark wooden hut,
with a whitened table in the centre, and a mirror and lens in the apex
of the roof. In fig. 70 we have a section of a camera obscura of this
kind. The mirror and lens at the top of the apparatus are made to
revolve, so as to bring every part of the landscape into view in turn. A
camera obscura in a position commanding a view of moving objects, such
as ships sailing to and fro, or the busy streets of a populous town, is
an unending source of amusement, and may be easily and cheaply
constructed.
The camera obscura has been much utilized for taking hasty but exact
sketches of various places. For this purpose it is made very light, and
mounted on three legs carrying at their junction a flat table, whereon
is placed the paper to receive the drawing. The tripod is covered with a
black curtain, which, falling over the artist, effectually excludes all
the rays of light except those which pass through the lens and are
reflected downwards by the mirror. In the better kind of apparatus the
mirror is replaced by a prism, which throws a clearer image than a
mirror upon the screen.
It is on these properties of the camera obscura that the art of
photography was founded. Everybody who saw the beautiful images formed
by this instrument was struck with the idea that by some means or other
they could be fixed on paper. After numberless attempts the
long-wished-for goal was at length arrived at; and now optics, aided by
chemistry, is enabled to depict for us natural objects of every kind,
from the distorted limb of the hospital patient to the beautiful forms
of the queens and empresses of the world—from the tiniest animalcule to
the great sun itself, who is compelled by the might of science to paint
his own portrait for us with all his faults and imperfections.
The lenses used for photographic purposes have only reached their
present state of perfection after ceaseless labours of the philosophers
and opticians of all countries. At first only a single lens was used,
but it was found that the rays which exercised a chemical action did not
meet in the same point as the rays of light, for it must be remembered
that it is not the light we see that acts upon the substances used in
photography, but another influence, known as actinism. It was also found
that a single lens would not give a flat picture when the whole of its
aperture was used, the edges of the image being always blurred and
indistinct. This latter defect was found to be partially obviated by
decreasing the opening, but this remedy shut off the light and prolonged
the process. Gradually these two defects were removed, and now every
photographer, no matter how humble, is possessed of a lens capable of
taking a clear picture, every detail of which is perfectly distinct and
faithful.
The camera lucida bears a great analogy to the camera obscura in the
purpose for which it is used, though not in the principle on which it is
constructed. It is employed, like the preceding instrument, for
obtaining faithful copies of a landscape, a building, or even of another
drawing. It was invented by Dr. Wollaston, in 1804, and consists of a
little four-sided prism, of which fig. 71 is a section.
[Illustration:
FIG. 71.—Section of Camera Lucida.
]
The angle at A is a right angle; the angle B measures 67½°, the angle C
135°, and the angle D is, of course, equal to B. It is mounted on a
sliding foot, so that it may be raised or lowered at will, or turned in
a horizontal direction. The path of the rays in this case is easy to
follow, the object to be copied being placed at L, and the eye at I. On
looking downwards the image of the object to be drawn is seen on the
paper; and if the eye is placed so that the edge of the prism will just
cut the pupil in two, the paper and pencil will be seen at the same
time. It will be seen from the diagram, that the rays proceeding from L
strike on the surface A B at right angles, and, being then reflected
from C B, pass upwards again to point E. The direction of the rays is in
reality a little more complicated than this. In the case of distant
objects it is impossible to see both the object and the pencil at the
same time; a lens is sometimes introduced at I to modify this defect.
The original instrument has also been modified by the introduction of a
triangular prism, in conjunction with plates of coloured glass, but the
difficulty of rendering the image and the paper of the same strength is
very great. The instrument is also hard to use, from the additional
difficulty of always keeping the head in the same position, for the
least movement from left or right is sufficient to throw the whole
drawing out.
A simple camera lucida may be made out of a small piece of
looking-glass, mounted at an angle of 45°, or half-way between the
horizontal and the perpendicular. If this be turned towards the drawing
or view to be copied, and the left eye applied to the mirror, the image
of the object will be seen on the paper below, and the pencil may be
guided with the right. The proper use of this simple little instrument
depends in a great measure upon the focus of each eye being the same.
The light falling on the paper, too, requires very careful adjusting,
otherwise the brighter object will eclipse the other. It is a good plan,
too, to whiten the pencil or pen used, so that it may not so easily be
lost when drawing the brighter parts of the object. We have seen
excellent drawings made from plants by means of a little instrument of
this kind, which simply consisted of a piece of looking-glass inserted
in a cork stuck in a glass bottle.
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