Page:Optics.djvu/149

 pierced through their centers to give a passage to the light. By this means, a small object being placed at $Q$, a first reflexion at $A$ would produce an image at $q$, which being farther from the center of the surface, would be larger than the object, but a second reflexion at the surface $B$ sends back the light which is proceeding to form this, and thus throws the image to $q_{2}$ still farther magnified; there an eye glass receives the rays, and transmits them with the proper divergence for distinct vision.

A small screen is placed at $c$ to prevent the rays from coming directly from the object to the lens.

163. This is merely a sort of magic lantern, in which the light of the Sun, reflected by plane mirrors, and condensed by lenses, is thrown on a minute transparent object, of which a magnified image is formed by means of a lens.

In the figure, the object to be exhibited is placed near the focus of the first combination of lenses, so as to be entirely enlightened by the rays coming through them, and not to be burnt, which would be the case, were it exactly at the focus.

164. This instrument consists of a plane mirror, which is made to revolve by clock-work about an axis parallel to that of the heavens, so as to reflect the Sun's light constantly in one same direction, during the course of the day.

It is found very useful in many optical experiments where a small pencil of solar light admitted into a darkened room, is to be subjected to reflexion, refraction, dispersion, &c.

It is a very convenient appendage to the solar microscope, which is the reason of its description being inserted here.

165. Generally speaking, the Telescope is in construction analogous to the compound microscope: the only difference is, that