PHOSPHORESCENCE SPECTRA. 



59 



coil. The curves of figure 45, for example, were obtained with a rela- 

 tively high vacuum, whereas those of figure 49 were obtained with a 

 very low vacuum, so that the decay was comparatively rapid and there 

 was only a suggestion of the beginning of the second process in the 

 position of the last point observed. Slight changes in temperature, 

 such as were produced when the liquid air fell below the line of the 

 crystal, were found also to produce changes in the initial brightness and 

 rate of decay. 



too 



25 



50 75 



SECONDS 



FIG. 48. 



too 



To determine whether the excitation produced any secondary change 

 in the crystal, which persisted after the phosphorescence had dis- 

 appeared, so that there would be a progressive building up of the 

 phosphorescence, excitations were made of equal length, repeated at 

 as nearly equal intervals as decay observations permitted. Figure 49 

 shows that, at a fairly low cathode vacuum, an excitation of 20 seconds, 

 repeated at approximately 1 -minute intervals, produced identical 



n 



aoo 



too 



10 



soo 



ri 



100 



to 



SECONDS 

 FIG. 49. 



100 

 SECONDS 



FIG. 50. 



250~ 



decay curves. The same effect is shown in figure 50 for a much longer 

 period of decay. When the time between excitations was short as 

 compared to the time and strength of excitation, there appeared to be 

 a progressive change, as indicated in figure 51. 



