18 LIGHT. 



in the figure, the central point being supposed to be 

 a star or other source of light. 



22. DIVERGENCE OF 

 Explain the 



divergence of LIGHT. By the diver- 



rays of light. gence of rayg Qf Hght 



is meant that they spread them- 

 selves over more space, the further they proceed from 

 their source. This is illustrated in the figure, where 

 the light of a candle is represented as passing through 

 a window, and illumining a larger space on the opposite 

 wall. 



23. LAW or DIVERGENCE. When the dis- 



Give the law of . 



divergence,and tance is doubled, the surface that light 

 illustrations. win coyer ig qua d rU pled. This is also 



illustrated in the figure. The wall being twice as far 

 from the candle as the window, the light covers four 

 times the surface. If the distance of the wall were 

 three times that of the window, the surface covered 

 would be nine times as large as the window ; if four 

 times, the surface covered would be sixteen times as 

 large. It is evident from these figures that the surfaces 

 covered, increase as the squares of the distances. The 

 light, of course, diminishes in intensity in the same 

 proportion, as it is thus spread over greater surface. At 

 four times the distance, it has only one-sixteenth the 

 intensity, and so on. 



24. REFLECTION OF LIGHT. If a ball of 



Explain the . 



reflection of ivory or other material be thrown perpen- 

 dicularly against any hard plane surface, it 

 will return in the same line ; if it be thrown obliquely, 



