Page:The wonders of optics (1869).djvu/253

 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