Page:Popular Science Monthly Volume 11.djvu/681

Rh directly over this spot, and held in position by the wooden handle with a piece of wax. A strip of paper, filling the distance from A to B, and four inches wide, is held upright between the cards, with the bottom resting on the mirror. The edge of this is marked with a pencil at the hole A, and again at the needle-point. A straight line joining these marks will form an angle at the bottom of the paper that is identical with the angle of incidence. By reversing the ends of the paper, and comparing this line with one from B to the needle, both will be found alike. The angles of incidence and reflection agree. In regard to the reflection of light. Prof. Mayer remarks:



Fig. 4 shows the effect of particles in scattering the light. A clean glass jar stands upon a black cloth laid on a table in a dark room, and over its mouth rests a postal-card having a slit in it one inch long and one twenty-fifth of an inch wide. A beam of light enters the room from one side, and is thrown downward upon the postal-card by a