1875] 



WRITINGS OF JOSEPH HENRY. 



487 



by L and L' respectively. Let the intensity of the candle 

 end equal one candle, while that of the lamp is n candles. 

 Then, since the illumination of the screen varies directly as 

 the intensity and inversely as the square of the distance, we 

 have the following proportion : 



1 n 



L: L' ::—^\ . _^ a , and when L = U we have (a — xf =nx^ 

 whence x = , ,',--. For convenience of using this formula 



l/n— 1 



l + V/1 



it is best to change its form into x = a 



The following table has been computed by calling the 

 length of the scale 100 and assigning successive integral 

 values to n, from 1 to 100. The column A shows the value 

 of a — X for each assumed value of n : 



Table of distancea and candle-powers. 



