296 WELLS'S NATURAL PHILOSOPHY. 



Whea rays of licrht radiate from any lumin- 



Explain the i i .1 t r \-i 



divergence of ous Docly, tliGy aiver2;e irom one another, or 



rays of light. , J5 J ^ J 



they spread over more siDace as they recede 

 from their source. 



Fig. 230 represents the manner of the diverg- Fig. 230. 



ence. 



What is the law Thc siirfaces covered, or 

 «f divergence? iHuminatcd by rays of 

 light diverging from a luminous cen- 

 ter, increase as the squares of the 

 distances. 



Thus, a candle placed behind a window will illuminate a certain space on 

 the wall of a house opposite. If the wall is twice as for from the candle as 

 from the window, the space illuminated by it will be four times as large as 

 the window. If the wall be removed to three times the distance, the surface 

 covered by the rays of light will be nine times as large, and so on. 



A collection of radiating rays of light, as shown in Fig. 230, constitutes 

 what is called a "pencil of light." 



A thousand, or any number of persons, are able to see the 



great numher same object at tho same time, because it throws off from its 



of persons able surface an infinite number of rays in all directions ; and ono 

 to see the same -^ ' 



object at the person sees one portion of these rays, and another person 

 same time? another. 



Any number of rays of light are able to cross each other, in the same space, 

 without jostling or interfering. If a small hole be made from one room to 

 another through a thin screen, any number of candles in one room will shino 

 through this opening, and illuminate as many spots in the other room as there 

 are candles in this, all tlieir rays crossing in tho same opening, without hinder- 

 ance or diminution of intensity ; just as sounds of different character proceed 

 through the air and communicate to the ear, each its own particular tone, 

 without materially interfering with each other. 



Eays of light which continually separate as 



When are rays *' "^ . ^ x 



Baid to be di- they procccd irom a luminous source, are called 

 verfing; and Diverging Rays. Rays wliich. continually ap- 

 *"*" ^ proach each other and tend to unite at a com- 



mon point, are called Converging Rays. Rays which move 

 in parallel lines, are called Parallel Rays. 

 What is a 645. When rays of light, radiated from a 



shadow? luminous point, through the surrounding 

 space, encounter an opaque body, they will (on account 

 of their transmission in straight lines) be excluded, from 



