< Page:Popular Science Monthly Volume 11.djvu
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SIMPLE EXPERIMENTS IN OPTICS.

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:

"The clouds, the water, the grass, rocks, the ground, buildings, the walls inside, clothing and furniture, and everything we can see, reflect light in every direction again and again, and thus it is that all spaces, without and within, are filled with light so long as the sun shines. At night the sun sinks out of sight, and still it is light for some time after, for the sunlight is reflected from the sunset-clouds and the sky. Sometimes, upon a summer's day, when broken clouds partly hide the sun, you will see long bars of dusky light streaming from openings in the clouds. These long bars are beams of sunlight shining upon dust and fine mist floating in the air, and we see them because each speck and particle reflects light in every direction."
Fig. 4.—Light reflected by Floating Particles.

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

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