232 



FLUORESCENCE OF THE URANYL SALTS. 



the rear disk, while the opaque sector prevented the light from coming through 

 the phosphoroscope to the eye. As the disk was revolving at a high speed, 

 the light was quickly stopped from passing to the crystal by the interposition 

 of an opaque sector of the rear disk. A small fraction of a second later the 

 continued rotation brought an open sector of the front disk in line with the 

 crystal and the eye, thereby allowing the phosphorescent light to be viewed 

 on a dark field. The rotating parts were neatly mounted in a brass drum 

 and driven by a crank through a system of gears. With such a type of 

 phosphoroscope Becquerel detected the glow of the platino-cyanides only 

 0.003 of a second after excitation. 



It is evident that to view opaque specimens, the excitation can not be 

 directly behind the specimen; hence Becquerel 1 devised a phosphoroscope 

 possessing only one rotating disk, figure 2, both diagrams with three openings, 

 K, L, and R, arranged 120 apart. The disk revolved on a vertical axis 

 between two fixed openings, 180 apart, the exciting light from X passing in 

 through one of these two openings and the luminescent light passing out from 

 the specimen P, upper diagram, through the other opening to the observer 

 a fraction of a second later. In figure 2 the sector is shown, in elevation, at 

 such a position that the open sector R admits exciting light to P, and it is 

 evident that an opaque sector is at that time to be found at S. On the other 

 hand, when S is opened by the passage of an open sector, the open sector R 

 has passed out of the line XP and an opaque sector is interposed. 



FIG . 2. 



E. Wiedmann 2 devised a phosphoroscope consisting essentially of a hollow 

 brass drum fitted with a collimator and lens and revolving disk for the ad- 

 mission of the exciting light. The exciting light passed intermittently through 

 the open sectors of a sectored disk and the specimen, which was mounted 

 within the drum, was thus intermittently excited. For the purpose of view- 

 ing opaque specimens and liquids at a later phase, the revolving disk carried 

 on the periphery a band with open sectors for excitation; thus the path of the 

 phosphorescent light was at right angles to that of the exciting light. Wiede- 

 mann constructed driving-gears with a ratio of 1,000 :1 and hence obtained a 

 rotary speed as great as 140 revolutions per second. Either the unassisted 

 eye or a spectrometer was employed to view the phosphorescence. Stray 



1 E. Becquerel, Annales de Chimie et de Physique (3), v. 55, p. 80. 1859. 



2 E. Wiedmann, Wiedmann's Annalen, vol. 34, p. 446. 1888. 



