SIMPLE EXPERIMENTS IN OPTICS. 



66 1 



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 

 5, 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 pen- 

 cil 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 withiu, are 

 filled with light so long as the sun shines. At night the sun siuks out of sight, 

 and still it is light for some time after, for the sunlight is reflected from the sun- 

 set-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 open- 

 ings 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 si)eck and par- 

 ticle reflects light in every direction." 



Fig. 4. Light reflected bt Floating Particles. 



Fig. 4 shows the efiect 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 



