PHOTOGRAPHIC STUDIES OK LUMINESCENCE. 



129 



that the shutter operated in a small part of a second. The length of 

 excitation and decay could be increased or decreased together by changing 

 the speed of the motor, while either the length of excitation or the length of 

 decay could be varied alone by rearrangement of parts of the controlling 

 devices. The total time of exposure of the photographic plate to the phos- 

 phorescence light was made equal to the length of exposure of the acetylene 

 spectra. 



With the apparatus described above, the photographic negative of 

 phosphorescence was obtained by light which varied in intensity, for the 

 phosphorescence decayed after the exciting light was closed off in the 

 manner characteristic of phosphorescent powders. However, if an energy 

 curve is obtained similar to that for fluorescence, it seems reasonably certain 

 not only that the energy curve does not change with decay, but also that it 

 does not change when the exciting light is shut off. 



44- 



.48 



.52 .56 



Fig- 137. 



.60 



Fig. 137 shows the energy distribution in the phosphorescence light of 

 Sidot blende. The powder was excited each time for 8.75 seconds and 

 allowed to decay 10.33 seconds. The powder was not exposed to infra-red 

 before each excitation. The effective length of exposure of the photo- 

 graphic plate was 60 minutes. Curves 1, 2, 3, and 4 show the transmitted 

 light of the photographic spectra corresponding to intensities of acetylene 

 equal to 6, 4, 2, and 1, respectively. Curve 5' shows the transmitted light 

 of the phosphorescence spectrum. Curve 5 shows the energy curve for the 

 phosphorescence light. 



It is seen that curve 5 is similar in form to the corresponding curves for 

 fluorescence. The phosphorescence spectrum, at least in the early stages 

 of decay, is thus the same as the fluorescence spectrum. 



DISCUSSION OE METHOD. 



The chemical change made in a photographic plate depends upon the 

 wave-length and intensity of the incident light and upon the length of 



