uS 



STUDIES IN LUMINESCENCE. 



.01 



.02 .03 



.06 



.07 .08 .09 



Iii Fig. 122, curve B, showing the decay under the action of the infra-red, 

 is the average of a number of determinations and the shape of the curve is 

 fairly definite. The data for curve A are, however, less reliable. There is 

 some doubt also as to the shape of the curve during the first hundredth of 



a second. It was neces- 

 sary to change the posi- 

 tion of the disk on the 

 shaft of the phosphoro- 

 scope in studying the 

 effect of infra-red and it 

 is possible that the zero 

 given may be incorrect, 

 i. c, the points on the 

 zero ordinate and per- 

 haps the one following 

 may be in the fluorescent 

 light, and therefore the 

 real phosphorescent de- 

 cay may have started at 

 some later time than 

 the zero given. This 

 possible error in the zero 

 only applies to the curves taken in studying the infra-red and in no way 

 affects the other curves. 



There is no question as to the effect of infra-red on the initial decay of 

 vSidot blende, but owing to the very slow decay the shape of the curves 

 may be more or less in error. 



DECAY CURVES FOR DIFFERENT WAVE-LENGTHS. 



In this determination the curves of decay were taken for different wave- 

 lengths of the phosphorescence spectrum of several substances. So far 

 as could be seen with the spectroscope all these spectra, at room temperature, 

 consisted of a single band, the location and extent of which in each case is 

 indicated in Table 18. 



.04 .05 



Seconds 



Fig. 122. 



Curves showing effect of infra-red on initial decay of Sidot blende. 

 Curve A, without infra-red, time of excitation 0.04 second. 

 Curve B, with infra-red, time of excitation 0.04 second. 



To determine the rate of decay of different parts of the band for any sub- 

 stance, two methods were used. One w T as to take a number of decay curves 

 at different wave-lengths of the band, and to plot a curve showing the inten- 

 sity, at a fixed time after excitation, for the different wave-lengths. In the 

 second method the mirror of the phosphoroscope was fixed so as to reflect the 

 light from the screen a certain time after excitation; then, by shifting the 

 telescope of the spectrophotometer, the intensity of the phosphorescence was 



