The wonders of optics/The camera obscura and camera lucida
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.
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
Fig. 70.—The Camera Obscura.
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 an-other drawing. It was invented by Dr. Wollaston, in 1804, and consists of a little four-sided prism, of which fig. 71 is a section.
Fig. 71.—Section of Camera Lucida.
The angle at A is a right angle; the angle B measures 67-1/2°, 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 intro-duction 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.