ON PHOSPHOROSCOPES. 



237 



drum (see fig. 7) whose periphery P is covered with a phosphorescent sub- 

 stance, provided with a source of excitation S mounted in a box A, and driven 

 by belt BB and pulley. The viewing collimator or photometer is indicated 

 at E. 



Kester 1 employed a device of this type, together with spectrometers and 

 radiometer, to study the relation of intensity of excitation to that of phos- 

 phorescence. 



B B, 



FIG. 7. 



Waggoner 2 mounted on a vertical shaft an iron drum 45 cm. in diameter. 

 This mass, being considerable, acted as a balance on the irregularities of the 

 motor speed. The exciting spark was so mounted that it could be moved 

 about the drum, thus enabling the observer to measure the brightness at 

 various times after the spark ceased. The periphery was painted, as before, 

 with the phosphorescent substance. The revolutions of the drum were auto- 

 matically recorded on a chronograph. With this device the spectrum of early 

 phosphorescence was studied with the aid of the spectrometer and decay 

 curves of the total visible radiation were taken. 



Nichols and Howes, 3 in the study of the phosphorescence of calcite, em- 

 ployed this type with a drum of 8.0 cm. diameter (see fig. 8). The eye-piece 

 E was arranged at 180 from the spark A, the spark thus being completely 

 hidden from the observer by the opaque drum Z). Effective screening was 

 added to prevent stray light from entering either face of the Lummer-Brodhun 

 cube L. B. and a filter F was interposed between the comparison lamp C 



1 Kester, Physical Review (1), vol. 9, p. 164. 



2 Waggoner, Carnegie Inst. Wash. Pub. No. 152, p. 119. 



3 Nichols, Howes, and Wilber, Physical Review (2), vol. 12, p. 350. 



1918. 



