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. I.)