LAW OF GRAVITATION. 35 



two, which is the square of the distance ; and, 

 if I put the screen at one third of the distance 

 from the lamp, the shadow on the large screen 

 would be nine times the size. Again, if I hold 

 this screen here, at B F, a certain amount of 

 light falls on it ; and if I hold it nearer the 

 lamp at E, more light shines upon it. And 

 you see at once how much exactly the quan- 

 tity which I have shut off from the part of this 

 screen, B D, now in shadow ; moreover, you see 

 that if I put a single screen here, at a, by the 

 side of the shadow, it can only receive one fourth. 

 of the proportion of light which is obstructed. 

 That, then, is what is meant by the inverse of 

 the square of the distance. This screen E is the 

 brightest because it is the nearest, and there is 

 the whole secret of this curious expression 

 inversely as the square of the distance. Now, 

 if you cannot perfectly recollect this when you 

 go home, get a candle and throw a shadow of 

 something your profile, if you like on the 

 wall, and then recede or advance, and you will 

 find that your shadow is exactly in proportion 

 to the square of the distance you are off the 

 wall ; and then if you consider how much light 

 shines on you at one distance, and how much 



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