42 



FLUORESCENCE OF THE URANYL SALTS. 



A lateral strip of the phosphorescent salt 1 cm. wide was excited by 

 sparks from a single spark-gap between zinc terminals and measure- 

 ments of the brightness were made at various times after the close of 

 excitation. The necessary conditions were attained by shifting the 

 disk successively through small angles, so as to vary the interval 

 between excitation and observation. The time could be estimated 

 with sufficient accuracy by noting the instantaneous positions of the 

 disk for each adjustment, as given by the strictly synchronous illumina- 

 tion due to the spark. 



zx 



FIG. 32. 



The arrangement of the apparatus is shown in figure 32, in which P 

 is the phosphorescent surface, DD the sectored disk, L. B. the Lummer- 

 Brodhun cube of the photometer, E the eyepiece, S a color-screen and 

 matte translucent plate, C the comparison lamp which traveled along 

 the track of an optical bench. The cross at Z indicates the position 

 of the spark-gap. 



In table 15 relative intensities 7, the reciprocals l/A/zT and times T 

 after excitation are given. Figure 33 shows the relations between I 

 and T, and 1/VFand T respectively in the usual manner. 



TABLE 15. 



As appears from the table and curve ABC, figure 33, this substance 

 exhibits a remarkably rapid decay, falling in the interval between 

 0.0005 second after close of excitation and 0.003 second to less than 

 three-thousandths of its intensity at the beginning of that interval. 

 To show the degree of accuracy with which the lower intensities were 

 observed, the portion of the curve BC is reproduced with ordinates 

 magnified ten times B'C'. _The results are likewise plotted in the cus- 

 tomary manner with 1/Vl as ordinates (curve DEF), and this brings 



