236 



FLUORESCENCE OF THE URANYL SALTS. 



per second. By means of a small set-screw the disk could be clamped at 

 various positions on the shaft, corresponding to various times in the decay 

 process. Without the star-wheel SS the discharge of the condenser KK 

 produced an exciting spark at E, which, with the aid of a revolving mirror, was 

 found to consist of a pilot-spark, followed by five or six smaller sparks; hence 

 the zinc star-wheel SS was mounted on the shaft to reduce the discharge to 

 one spark. By experimenting with small and large capacities it was found 

 that resonance must be recognized, and the proper amount of capacity to 

 produce a regular, strong spark was finally discovered. The measurements 

 of time were read with the aid of the light yielded by the exciting spark by 

 noting the position of the edge of the sectors on a protractor mounted rigidly 

 in front of the machine. The range of times accurately measureable include 

 those from 0.0001 to 0.0040 second by 0.0001-second steps. The photometer, 

 spectro-photometer, camera, and spectrograph have been successfully used 

 with this instrument. 



FIG. 6. 



In their preliminary study of the cathodo-phosphorescence of the rare 

 earths, Nichols, Wick, and Wilber 1 employed a phosphoroscope with a disk 

 mounted on the shaft of a motor. The primary of an induction coil was 

 interrupted by a plunger attached to a crank on the shaft of the motor. The 

 plunger, once in a revolution, dipped into the mercury cistern, while the 

 opaque portion of the disk hid the specimen from view, and as a result the 

 specimen was excited by the discharge of the secondary coil through a cathode- 

 ray tube, much after the manner of Crookes's device. 



The change in time between excitation and observation was accomplished 

 by changing the speed of the motor, and an ammeter in the field circuit was 

 calibrated to measure the angular velocity. It is evident that with only one 

 excitation per revolution only one open sector could be used. 



The second type of phosphoroscope includes those instruments by means 

 of which the specimen is constantly excited and constantly viewed at a later 

 time. Such a form had its origin with E. Becquerel. 2 This form was used for 

 lecture demonstrations by Tyndall. 3 In its simplest form it consists of a 



1 Nichols, Wick, and Wilber, Physical Review (2), vol. 14, 1919. 



2 Becquerel, E., Annales de Chimie et de Physique (3), vol. 62, p. 5. 

 ' Tyndall, see Lewis Wright, "Light," p. 152. London, 1882. 



1861. 



