these rays; and as we always see objects in the direction taken by the luminous ray at the moment it enters the eye, we fancy we see objects before us that are really behind, or on each side of us. For instance, the ray starting from the left foot of the young lady in the figure is reflected from the point indicated on the surface of the glass, but the eye does not stop here, but sees the foot at an equal distance beyond the mirror.
The same thing takes place, not only with glass, but with all substances having polished surfaces. Still water, which to all intents and purposes has a polished surface, reflects the objects within its range as perfectly as a mirror.
The preceding observations apply to all plane reflecting surfaces; but there are other sorts of mirrors, whose effects are of a more interesting nature, and which we must hasten to describe—we allude to those whose surfaces are either convex or concave.
Curved mirrors are made of a great variety of shapes, but for the present we shall only describe those which are spherical. Spherical mirrors may of course be either concave or convex.
Fig. 18.—Concave Mirror.
Suppose the arc M N (fig. 18) to be movable round the point O, this revolution will describe the surface of the mirror. The central point C of the hollow sphere
