PHOTOGRAPHIC STUDIES OF LUMINESCENCE. 



*33 



subsequent figures ordinates represent intensities of transmitted light. 

 These two curves are consistent as regards general shape and the position 

 of the points of minimum transmission. Since intensities of transmitted 

 light are plotted, the minima of the curves correspond to the maxima of 

 intensities of the spectra. Inspection will show that curve B is not the 

 exact duplicate of curve . 1 ; the ratios of the ordinates at long wave-lengths 

 are not the same as at shorter wave-lengths. This lack of similarity did 

 not exist in many of the comparisons and was, quite likely, due either to 

 eye fatigue or else to the gradual warming up of the plate under successive 

 exposures to the light which illuminated the slit. 



Fig. 139 shows three curves corresponding to the decay of phosphores- 

 cence. In curve A, the Sidot blende was excited for 9.75 seconds and the 

 plate was exposed for 8.25 seconds immediately after excitation. The 

 powder was exposed for about 1 minute to infra-red rays and the process 

 was repeated over and over until a sufficient exposure had been secured. 



SO 



.54 .56 



Fig. 139- 



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V 



2 

 I 



.38 



AZ .4-6 



Fig. 140. 



50/< 



In curve B the powder was excited for 21 seconds, allowed to decay for 14 

 seconds, then the plate was exposed for 15 seconds. In curve B the x-axis 

 is raised so that curve B does not intersect curve A. In curve C the exci- 

 tation was 5.5 minutes, the decay 1.5 minutes, and the plate was then 

 exposed for 1 minute. The total time of exposure to produce the negative 

 from which curve C was made was 72 hours. The negative was faint and 

 the film was somewhat fogged, but remeasurements on the film always 

 gave approximately the same curve. In fact, curve C is the average of sev- 

 eral remeasurements. The minima of these curves occur at wave-length 

 M = 0.555 0.003. 



The bend in the curve of decay of the phosphorescence of Sidot blende 

 occurs between 10 and 20 seconds after the end of excitation and is more 

 pronounced the longer the excitation. Hence curve A in Fig. 139 is due 

 chiefly to the light corresponding to the decay before the bend, while curve 

 B corresponds to conditions near the bend, and curve C corresponds to 

 conditions far beyond the bend. Or, in terms of the two-band theory, 

 curve A corresponds mainly to band 1 ; curve B, to bands 1 and 2 ; and curve 

 C to band 2 almost entirely. This set of curves shows no change in the 

 maxima of the spectra, and indeed no set was obtained which showed any 

 appreciable change. 



