REFLECTION OF LIGHT. 379 



of the body that diffuses it. A perfect mirror would be 

 invisible. Luminous bodies are visible on account 

 of the light that they emit; non-luminous bodies 

 are visible on account of the light that they dif- 

 fuse. 



(a.) If a beam of light fall upon a sheet of drawing paper, it 

 will be scattered and illuminate a room. If it fall upon a mirror, 

 nearly all of it will be reflected in a definite direction, and intensely 

 illuminate a part of the room. Place side by side upon a board 

 a piece of black cloth (not glossy), a piece of drawing paper and a 

 piece of looking-glass. In a darkened room, allow a beam of sun- 

 light to fall upon the cloth and notice the absorption. Let it fall 

 upon the paper, and notice the diffusion of the light and its effects. 

 Let it fall upon the looking-glass, and notice the regular reflection 

 and its effects. Move the board so that the cloth, paper and glass 

 shall pass through the beam in quick succession, and notice the 

 effects. 



(&.) In the darkened room place a tumbler of water upon a table ; 

 with a hand-mirror reflect a sunbeam down into the water; the 

 tumbler will be visible. Stir a teaspoonful of milk into the water, 

 and again reflect the sunbeam into the liquid ; the whole room will 

 be illuminated by the diffused light, the tumbler of milky water 

 acting like a luminous body. 



593. Invisibility of Light. Rays of light that 

 do not enter the eye are invisible. A sunbeam 

 entering a darkened room is visible because the floating 

 dust reflects some of the rays to the eye. If the reflecting 

 particles of dust were absent the beam would be invisible. 



(a.) Take any convenient box, about 60 cm. (2/.) on each edge, 

 provide for it a glass front, and, at each end, a glass window about 

 10 cm. (4 inches) square. Place it on a table in a darkened room, 

 and, with the heliostat, send a solar beam through the windows. 

 Standing before the glass front of the box, this beam may be 

 traced from the heliostat to the box, -through the box and beyond 

 it. Open the box, smear the inner surfaces of its top, back and 

 bottom with glycerine, and close the box air-tight. Allow it to 

 remain quiet a few days ; the dust in the box will be caught by 

 the glycerine and the confined air thus freed from particles capable 



