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.