Page:Optics.djvu/141

 to it at $D$. It will easily be seen that any ray of light entering the prism perpendicularly to the face $CD$, and therefore unrefracted, will be reflected by the faces $CB$, $BA$, so as to emerge perpendicularly through $AD$, and therefore appear to come from under the prism. The consequence of this is, that if the instrument be turned towards a distant object, the rays proceeding from it, emerging after the reflexion near $A$, will appear to form an image of the object on a paper placed under the prism, where it may be traced out with a pencil.

There are sometimes attached to this instrument, a concave lens, which may be placed in front of $CD$, and a convex one which is placed under the point $A$, by means of which the apparent distances of the paper and the image may be equalized, so that both may be seen distinctly at the same time, the concave lens bringing the image to the paper, or the convex throwing the paper to a distance, according as the sight of the person, using the instrument, is short or long.

156. In this instrument, or toy, or whatever it may be called, an image of a distant object, either stationary, or moving, is produced by means of a convex lens, plane mirrors being added either to reflect the rays to the lens, or to throw the image in any required direction, so as to fall on a sheet, or other object placed for its reception.

In Fig. 165, which represents the camera obscura shown by the Plumian Professor, in his Experimental Lectures, $A$ is the lens; $BD$ the plane mirror placed at half a right angle to its axis, so as to throw the refracted rays from the horizontal direction into the vertical one; $CD$ is a plate of ground glass, to receive the image on its under side; $EF$ is a lid with a curtain attached to it, which the spectator puts over his head, to exclude all extraneous light.

Fig. 166, represents another form of this instrument. Here the lens $A$ is horizontal, and the external rays are directed through it by the mirror $B$ placed as before, at an angle of 45 degrees to its axis. The image is received on a surface $C$, which is curved so as to fit it, and viewed through an aperture at $D$.