252 Captain Abney and Major-General Festing. [Dec. 8, 



Let 



W be the watts, 

 c „ current, 

 p „ potential, 

 y ,, intensity of light ; 



all other letters being constants. 



In order for the curve of intensity to be parabolic 



(i.) W — m — n*/y, 



m being the number of watts at which the vertex of the parabola 

 lies. 



From the equations given in the paper already referred fco (' Phil. 

 Mag.,' September, 1883), where 



c = ap + bp Sl2 , 



the above equation (i) may be written — 



p 2 (a-\-bp i )—m = n*/y ; 



when p is fairly large this becomes — 



(ii) p 2 — h = Tv^/y approximately. 



Similarly it may be shown that — 



(iii) c 2 — s = t*/y approximately. 



The following tables will show the application of (i). It must be 

 understood that the measures of current and potential are not given 

 in amperes and volts, and that as a consequence the watts are only 

 represented by watts x a constant. The first three tables show the 

 exactitude of the method of measurement where the resistance is 

 altered, and the fourth table the exactitude when the rotating sectors 

 are altered. 



