The principle of reflection is illustrated in Fig. 1, in which a beam of the sun's rays enters through the shutter of a dark room, strikes upon a polished plane surface, and is reflected across the room in an opposite direction. The entering beam, A B, is called the incident ray. The vertical line, B D, is termed the normal and the beam B C,
the reflected ray. The angle A D B, contained between the incident ray and the normal, is termed the angle of incidence; and the angle C B D, contained between the corresponding reflected ray and the normal, is called the angle of reflection. The reflection of light by polished surfaces, as in this case, is governed by two laws: 1. The incident ray, the normal, and the reflected ray, are always in the same plane; and, 2. The angle of incidence is always equal to the angle of reflection.
This is an example of what is known as regular reflection, but there is another kind of reflection in virtue of which bodies, when illu-