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



the applicability of the parabolic formula was given, for white light, 

 but by itself it was hardly conclusive. We, therefore, conducted a 

 series of experiments to ascertain if our anticipations were correct. 



An incandescence lamp was selected as a standard lamp, through 

 which a fixed current was maintained. This we used instead of a 

 standard candle or other variable light. We then selected a second 

 similar lamp, of which to measure the light when currents of various 

 strengths were passed through it. 



The shadow and grease-spot methods were both experimented with, 

 the former being perhaps the most exact. Whichever method is, 

 however, employed, it was inexpedient to move either lamp towards or 

 from the source, or to vary the distance of the source from the lamp, 

 as the carbon filaments show more or less illuminating surface to the 

 screen according as they are close or distant from it. It therefore 

 became necessary to adopt some other plan for altering the intensity 

 of the light falling on the source from the comparison lamp. 



In the Rumford (shadow) method, fig. 1 will give the general idea 

 of the arrangements. 



Fig. 1. 



The shadows ca st by the rod D from the two sources of light, 

 and L n , were made just to touch each other, on the white screen SS, 



