DECAY OF PHOSPHORESCENCE IN SIDOT BLENDE. 



6 9 



SUMMARY. 



The most important points brought out by the experiments here de- 

 scribed may be briefly stated as follows : 



1. Form of Decay Curve. The curve obtained by plotting the values of 

 7~* as ordinates and the corresponding values of / as abscissas is a straight 

 line for small values of / ; it changes to a curve concave toward the axis of t 

 as / increases; but for still larger values of / the relation between Z~* and 



/vr 



500 



400 



300 



200 



100 



10 20 30 40 



Seconds 



50 



60 



100 



200 300 



Seconds 



400 



Fig. 61. Willemite. 



Excited by a spark between iron 

 terminals. The curves were taken 

 in the order indicated by the let- 

 ters and with the excitations 

 stated below: 



Curve A, 0.7 sec; B, 3.4 sec; C, 

 9.0 sec; D, 60.0 sec; E, 3.9 sec: 

 F, 1.0 sec. 



Fig. 62. Balmain's paint. 



The curves were taken in the order 

 indicated by the letters, and with 

 the following excitations: 



Curve A, 5.9 sec; B, 12.0 sec; C, 

 26.0 sec. ; D, 42.0 sec; E, 6.4 sec 



/ is again linear, and remains so until I becomes too small to measure. In 

 other words, the decay curve, when plotted in this way, consists of two 

 straight portions which gradually merge into one another. 



2. Effect of Duration and Intensity of Excitation. Not only the intensity 

 of phosphorescence, but also the form of the decay curve, is dependent on 

 the intensity and duration of excitation. The slant is altered in each of 

 the straight parts of the curve by changing either of these two factors in 

 the excitation. 



