DIVERGING EATS. 47 



dimension each way, making the spot illuminated by the 

 light on the sheet four times as large as the hole through 

 which the light came. 



"We might know that it must be so," said Lawrence, 

 " since the rays of light proceed in straight lines, and so 

 diverge from each other at the same rate at every distance. 

 It follows from this that, since that portion of rays which 

 pass through the square hole have diverged from each 

 other one inch in passing one foot from the source and 

 those that pass through the hole must do exactly that 

 they will diverge from each other two inches in passing 

 through two feet. This will, of course, make the bright 

 spot twice as long, and also twice as wide, for the diver- 

 gence is the same in both dimensions, and thus the bright 

 spot will be four times as large as the opening through 

 which the light passed to make it. By the same reasoning, 

 if the distance were three feet, it would be nine times as 

 large, for the bright spot would contain three rows of 

 spaces as large as the opening, and there would be three 

 spaces in each row. If the distance were five feet, the il- 

 luminated space would be twenty-five times as large as the 

 opening. And so in all cases. The space illuminated by 

 any particular portion of the light from any point will be 

 as the square of the distance ; and as the intensity of the 

 light, supposing that none of it is lost, would be dimin- 

 ished just in proportion to its diffusion, the intensity upon 

 any given space will be inversely as the square of the dis- 

 tance:'' 



The principle is the same, whatever is the form of the 

 opening through which the light shines, whether square, 

 or round, or of any irregular figure. "Whatever the shape 

 of the opening may be, the surface that it illuminates will 

 be of the same shape that is, mathematically similar, and 

 it must be enlarged, so far as it is enlarged at all, in two 



