6 4 



STUDIES IN LUMINESCENCE. 



they strike the vertical axis. From the intercept thus determined, which 

 is equal to h~K the initial intensity can be computed. From the results 

 obtained in this way from the data shown in Fig. 52, curve A in Figs. 53 

 and 54 has been plotted. In these, two figures / has been plotted instead 

 of /-*. 



It will be noticed that the intensity of phosphorescence at first increases 

 rapidly with increased duration of exposure, but that after an exposure 

 of 2 or 3 minutes is reached there is little further change. The phospho- 



15 Mir 



Fig- 53- 



Effect of duration of excitation. Curve A shows initial intensity of phosphorescence (/) as a function 

 of the time of excitation. Curvesa.b, and c show the decay of phosphorescence after excitations of 15 

 minutes, 5 minutes, and 1 minute, respectively. 



rescence may be said to be saturated so far as the effect of duration of 

 excitation is concerned. Not only is the initial intensity unaltered by 

 longer excitation, but the form of the decay curve also remains constant, 

 as is indicated by curves a and b in Fig. 53. (These curves correspond 

 to curves F and E of Fig. 51.) 



We have also studied to some extent the influence of the intensity of 

 the exciting light upon the form of the decay curves. In order to vary 



Fig- 54- 

 Effect of duration of excitation. A portion of the same curve shown in Fig. 54 plotted to a larger scale. 



the intensity of the exciting light several metal stops were prepared, which 

 could be placed immediately in front of the mercury lamp. The apertures 

 of these varied from i mm. in diameter to the full size of the mercury-lamp 

 tube, namely, 15 mm. To determine the intensity of the exciting light 

 corresponding to each of these the following photometric method was 

 used: The phosphorescent screen was removed from the photometer and 

 a piece of white cardboard was put in its place. This being illuminated 



