1887.] 



On Photometry of the Glow Lamp. 



249 



and to fall within a rectangle cut out of black paper, which deadened 

 the light on the rest of the screen. Each lamp (L x and L n ) was in 

 connexion with an ampere-meter and volt- meter (A Y , A n and V l5 V n ).' 

 In front of L x (the comparison lamp) was placed an electromotor 

 which caused a pair of sectors of variable aperture to rotate between 

 it and the screen. 



Evidently two methods are open to equalise the illumination of the 

 screen from each source : — 



1st. Cutting off more or less light from L x . 



2nd. Varying the current in L n by means of the variable resistance 

 in the circuit. 



The first plan necessitates the opening and closing of the sectors 

 whilst rotating, and the second the alteration of the resistance, &c, 

 at will. Whichever method was adopted the lamp L x was brought to 

 a bright yellow glow, and the lamp L n had a current passed through 

 it which, when the minimum resistance was in circuit in R, produced 

 a brilliant white light. Such intense heat the filament would not be 

 able to stand for any considerable time. 



When measurements were, to be taken by the first plan the instru- 

 ment shown in the annexed diagram, fig. 2, was employed. A pair of 



Fig. 2. 



sectors, S (each of 90°), are mounted on a horizontal axis, a similar 

 pair, S 1? are carried on a short sleeve, to which are attached two 

 horizontal pins, passing through holes in the flange D of another 



