CATOPTRICS is that part of optics which explains the properties of reflected light, and particularly that which is reflected from mirrors.

As this and the other branches of Optics will be fully treated under the collective word, we shall, in the present article, 1st, Just give a summary of the principles of the branch, in a few plain aphorisms, with some preliminary definitions; and, 2dly, Insert a set of entertaining experiments founded upon them.

Sect. I. Definitions.

1. Every polished body that reflects the rays of light is called a mirror, whether its surface be plane, spherical, conical, cylindric, or of any other form whatever.

2. Of mirrors there are three principally used in optical experiments: The plane mirror, GHI, (fig. CXXXVI.) CATOPTRICS.

3. The point K, (fig. 2, 3.) round which the reflecting surface of a spherical mirror is described, is called its centre. The line KH, drawn from its centre perpendicular to its two surfaces, is the axis of the mirror; and the point H, to which that line is drawn, is its vortex.

4. The distance between the lines AG and BG, (fig. 1.) is called the angle of incidence, and the distance between BG and CG is the angle of reflection.