238 



FLUORESCENCE OF THE URANYL SALTS. 



and the cube to produce a color match with the reddish color of the phos- 

 phorescence. Although the angle between spark and observing photometer 

 remained constantly 180, the time of decay could be varied between 0.01 

 second and 3 or 4 seconds by changing the motor field or by throwing in a 



DISK PHOSPHOROSCOPE. 







FIG. 8. 



M 



FIG. 9. 



worm-gear drive. Two features were added to make the device more useful. 

 First, the phosphorescent substance was never painted on the disk, but on 

 brass rings which fitted neatly on the disk; second, the speeds were known by 

 reading on a galvanometer G the deflections produced by a current induced 



