BUNSEN'S PHOTOMETER. 



425 



nation of four candles on the other side. The illumi- 

 nating power of the combination being four times that 

 of the single candle, it will be found that the grease- 

 spot disappears when the four candles are twice as far 



from the screen as the single candle. In fig. 242 the 

 distances are 25 cm and 50 cm ; the squares of these dis- 

 tances are 625 and 2500 ; but 2500 : 625 :: 4 : 1, or 



2500 

 625 



