Page:A treatise on optics.djvu/15

(8.) Catoptrics is that branch of optics which treats of the progress of rays of light after they are reflected from surfaces either plane or curved, and of the formation of images from objects placed before such surfaces.

(9.) Any substance of a regular form employed for the purpose of reflecting light, or of forming images of objects, is called a speculum or mirror. It is generally made of metal or glass, having a highly polished surface. The name of mirror is commonly given to reflectors that are made of glass; and the glass is always quicksilvered on the back, to make it reflect more light. The word speculum is used to describe a reflector which is metallic, such as those made of silver, steel, or of grain tin mixed with copper.

(10.) Specula or mirrors are either plane, concave, or convex.

A plane speculum is one which is perfectly flat, like a looking-glass; a concave speculum is one which is hollow like the inside of a watch-glass; and a convex speculum is one which is round like the outside of a watch-glass.

As the light which falls upon glass mirrors is intercepted by the glass before it is reflected from the quick-silvered surface, we shall suppose all our mirrors to be formed of polished metal, as they are in almost all optical instruments.

Fig. 1

(11.) When a ray of light, A D, fig. 1., falls upon a plane speculum, M N, at the point D, it will be reflected or driven back in a direction D B, which is as much inclined to E D, a line perpendicular to M N, as the ray A D was; that is, the angle B D E is equal to A D E, or the circular arc B E is equal to E A.