REFRACTION OF LIGHT. 



393 



ECTION ill. 



REFRACTION OF LIGHT 



611. Preparatory. So far, we have considered only 

 that part of the incident beam that is turned back from 

 the reflecting surface. As a general thing, a part of the 

 beam enters the reflecting substance, being rapidly absorbed 

 when the substance is opaque and freely transmitted when 

 the substance is transparent. We have now to consider 

 those rays that enter a transparent substance. 



(a.) Procure a clear glass bottle with flat sides, about 4 inches 

 (10 cm.) broad. On one side paste a piece of paper, in which a circu- 

 lar hole has been cut. On 

 this clear circular space, 

 draw two ink-marks at 

 right angles to each 

 other, as shown in Fig. 

 292. Fill the bottle with 

 clear water up to the 

 level of the horizontal 

 ink-mark. Hold it so 

 that a horizontal sun- 

 beam from the heliostat 

 may pass through the 

 clear sides of the bottle 

 above the water, and no- 

 tice that the beam passes 

 through the bottle in a 

 straight line. Raise the 

 bottle so that the beam 

 shall pass through the 

 water, and notice that the 

 beam is still straight. FlG - 2 9 2 - 



In a card, cut a slit about 



5 em. long and 1 mm. wide. Place the card against the bottle as 

 shown in the figure. Reflect the beam through this slit so that it 



