PHOSPHORESCENCE SPECTRA. 



45 



I" 



80 



EXCITATION IN THE PRESENCE OF RED AND INFRA-RED RAYS. 



To determine whether red or infra-red rays have an effect on these 

 substances similar to that observed in the case of the phosphorescent 

 sulphides, a modification of the apparatus was made such that the 

 surface under examination could be subjected to the intense illumina- 

 tion obtained by focusing the crater of an electric arc upon it. A screen 

 of excellent ruby glass was interposed to cut off all but the longer waves 

 and observations were made through a screen quite impervious to red. 



Exposure to this source was found to affect measurably neither the 

 brightness of fluorescence nor of phosphorescence. Curves taken after 

 exposure to this source, those taken with the substance subjected to 

 it interruptedly throughout the run, and curves in the determination 

 of which readings were taken alternately with and without red light 

 were all identical with those taken in entire absence from such expos- 

 ures. The striking contrast 

 between this negative result 

 and the well-known effects of 

 infra-red radiation upon the 

 phosphorescence of the sul- 

 phides is notable. 



The observations already 

 cited, showing the complete 

 identity of the spectrum of 

 fluorescence with that of phos- 

 phorescence seemed to indicate 

 that the intensity would go 

 over from that of fluorescence 

 to that of phosphorescence 

 without discontinuity. This 

 conclusion was confirmed, 

 within the errors of observa- 

 tion, by measurements just 

 before and after the close of 

 excitation. The only previous 

 instances where this relation 

 has been experimentally established, so far as we know, are to be found 

 in Waggoner's 1 studies of phosphorescence of short duration and in 

 recent observations on the luminescence of kunzite. 2 



In view of the unexpected character of the decay curves for the phos- 

 phorescence of the uranyl compounds, the question arises whether the 

 rather unusual mode of excitation employed, i. e., periodically repeated 

 exposures, 120 times a second, to groups of sparks of high frequency, 

 might produce such a result, or whether the decay curves are character- 



1 Waggoner, Physical Review, xxvu, p. 209. 



2 Nichols and Howes, Physical Review (2), iv, p. 26. 



eo 



40 



20 



FIG. 35. 



