ON PHOSPHOROSCOPES. 



235 



in the box K and was visible to the observer, except when the two arms of the 

 rotating disk D intercepted the phosphorescent light. At such times one of 

 the points, P or P r , completed the circuit from B through A to K, allowing the 

 discharge of the condenser C to excite the specimen at K; the condenser had 

 been previously charged by coil S. Twice a revolution, then, the specimen 

 was excited and observed. 



FIG. 5. 



Nichols and Howes, 1 to study the phosphorescence of the uranyl salts, de- 

 vised a phosphoroscope of considerable precision. Except for the work of 

 Nichols and Merritt, Trowbridge, Ives, and a few others, the previously 

 mentioned students of phosphorescence have been content to measure the 

 intensity at two or three periods of decay, but the above-mentioned investi- 

 gators have taken many observations on one substance and established curves 

 of decay for each substance studied. For such measurements, refinements for 

 precluding measureable stray light and for accurately measuring the time 

 intervals and for maintaining constant speed are a necessity. The synchrono- 

 phosphorosope (see fig. 6) was so named because it employs the principle of a 

 sectored disk mounted on the axle of a synchronous motor A . C. This motor 

 was raised to synchronous speed by the direct-current motor D. C. The 

 transformer TT was attached to the same alternating-current terminals as 

 was the A. C. motor; hence the discharge of the condenser occurred as many 

 times per second as the number of wave-crests, i. e., 120. There were four 

 opaque sectors and four open sectors in the disk WW, and since the four-pole 

 machine turned at a speed of 30 revolutions per second, there were 120 eclipses 



1 Nichols and Howes, Science, n. s., vol. 43, p. 937. 1916. 



