58 STUDIES IN LUMINESCENCE. 



THE STUDY OF PHOSPHORESCENCE IN SI DOT BLENDE AND CERTAIN OTHER 

 SUBSTANCES DURING THE WHOLE PERIOD OF DECAY. 



In the experiments just described the phosphorescence of Sidot blende 

 was followed by the spectrophotometer for about 10 seconds after the 

 cessation of excitation. At the end of this period the intensity of phos- 

 phorescence had fallen to about 4 per cent of its original value. Although 

 it was possible to see the phosphorescent light in the spectrophotometer 

 for a much longer period than this, the extreme faintness of the field made 

 accurate measurements out of the question. In attempting to determine 

 the law of decay throughout a wider range we have therefore been com- 

 pelled to abandon the use of the spectrophotometer and to measure the 

 total light. 



Measurements of the total intensity do not, in general, afford a satis- 

 factory means of determining the law of decay of phosphorescence, since, as 

 has already been pointed out, the phosphorescence spectrum usually con- 

 tains two or more bands, which decay at different rates. We have already 

 referred to the difficulties that are met with in interpreting the results of 

 such measurements. In the case of Sidot blende the conditions are, how- 

 ever, relatively simple, for the phosphorescence of the green band is so much 



-R 



i= 



Fig. 46. 



more prominent, both in duration and in intensity, than that of the violet 

 bands that the presence of these does not appreciably affect the total 

 intensity. Measurements of the total phosphorescence of Sidot blende are 

 therefore practically measurements of the intensity of the green band only. 

 If all parts of this band decay at the same rate, measurements of total in- 

 tensity will give the same results as measurements taken at some particular 

 portion of the band. Our earlier observations have shown that all parts of 

 the green band of Sidot blende do decay at the same rate for at least 10 seconds. 

 It seems not unlikely, therefore, that this equality of rates will be maintained. 

 Although the present section deals chiefly with the phosphorescence of 

 Sidot blende, the results obtained in the study of several other substances 

 are also included. 



EXPERIMENTAL METHOD. 



The apparatus used is shown in Fig. 46. The white screen was removed 

 from a Lummer-Brodhun photometer and the Sidot blende screen, s, was 

 put in its place, the active surface of the screen being toward the left. The 

 screen was usually excited by a Lummer mercury lamp, L, of the "end on" 

 form. In some instances the carbon arc, and in other cases the spectrum 

 of the carbon arc, was used for excitation. The exciting light could be cut 

 off by means of a shutter, S, which at the same time made a record on a 



