Catoptrics

[Gr. katroptikos, from katoptron, a mirror; kata, and optomai, to see.] ( Optics.) That part of the science which treats of the reflection of light from the regularly formed surfaces of such bodies as water, gla , and the metals. The name of speculum, or mirror, is generally given to those substances which have a highly polished and regularly formed surfaco; but, in general use, the term speculum is applied to a metal reflector; and mirror, to a reflector made of gla , and coated with an amalgam of tin and mercury or a layer of pure silver. There are four kinds of reflectors used in optics, - the plane, the convex, the concave, and the cylindrical; and when a ray of light is incident upon any of these surfaces, its reflection is always governed by the same laws. These laws, which are the result of observation and experiment, form the groundwork of catoptrics. They are as follows: - First, that the incident ray, the normal to the surface at the point of incidence, and the reflected ray, are all in one plane; second, that the angle of reflection is always equal to the angle of incidence. - See MIRROR, OPTICS, REFLECTION, SPEC ULUM.