THE BOOK OF THE LANTERN. 



117 



up for a brief period, and then turned down to darkness 

 once more. But how must this period be measured ? In 

 order to answer this question I must call attention to a 

 certain optical law which I have attempted to put in the 

 form of diagrams : 



FIG. 38. 



Fig. 38 illustrates the manner in which the light rays 

 from a candle strike out all round it like the spokes of a 

 wheel ; but, for simplicity sake, the rays in one direction 

 only are shown. Let A represent our printing frame held 

 at 1 foot from the light source, and let us suppose that 

 at that distance the plate contained in it will require one 

 second's exposure. Now let us expose a similar plate at B, 

 which is 2 feet from the light source, what exposure will 

 it require? "Two seconds," the tyro will probably 

 answer ; but he would be wrong, the plate at double the 

 distance will require four times the original exposure ; at 

 treble the distance, nine times the original exposure. In 



