PHOSPHORESCENCE OF METALLIC SULPHIDES. 495 



slow. The total light effect obtained by a single excitation is in- 

 adequate for the proper exposure of the plate when used in the 

 camera in the ordinary manner. Indeed it was found in some pre- 

 liminary trials by Professor G. S. Moler and the writer that a plate 

 placed in immediate contact with a tube containing the phosphores- 

 cent sulphide as soon as practicable after intense excitation and 

 allowed to remain for several minutes or until nearly the whole of 

 the total light effect had been utilized, was decidedly under-exposed. 



On the other hand the color effects to be recorded change rapidly, 

 especially during the first few hundredths of a second after excita- 

 tion so that it was necessary to obtain the equivalent of a large num- 

 ber of successive short exposures each made at the particular time 

 after excitation for which the color record was desired. 



To this end a special form of phosphoroscope^ was constructed 

 by means of which the substance, enclosed in a flat tube of glass 

 about 10 cm. long and from i cm. to 1.5 cm. wide, was viewed 

 through a revolving disk with alternate open and closed sectors of 

 45° aperture. The disk was mounted on the shaft of an alternating 

 current synchronous motor on a 60-cycle circuit and excitation was 

 by a series of five sparks (£, Fig. i) between zinc terminals. 



The sparks were produced by the action of the secondary coil 

 of a small step-up transformer with condenser C, in the same circuit. 



! 



e: 



1 



T 



Fig. I. 



A spur wheel of zinc (5) was mounted on the shaft of the motor 

 and its four arms passed the two terminals of the sparking circuit 

 with an air space of about i mm. The wheel was adjusted so that 

 this passage coincided with successive crests of the alternating-cur- 



- See a paper presented by Nichols and Howes at the meeting of the 

 Am. Phys. Society, Washington, April, 1916, abstract Physical Revieiv (2), 

 VII., p. 586, 1916. 



PROG. AMER, PHIL. SOC, VOL LV, EE, JULY I9, I916. 



