6o 



STUDIES IN LUMINESCENCE. 



first 6 or 7 seconds this coefficient is practically constant, as was shown by 

 the results of our previous experiments. A change then begins which con- 

 tinues for about 20 or 30 seconds, after which the curves indicate a new 

 coefficient, smaller than before. This new coefficient, as is shown by the 

 constant slant of the curves of Fig. 47, remains constant during the rest of 

 the period of decay, or at least until the phosphorescence has become too 

 faint to measure. Whatever theoretical interpretation of the curves is 



Fig- 47- 



Effect of duration of excitation. 

 Violet end of carbon-arc spec- 

 trum used in excitation. The 

 curves were taken in the 

 order indicated by the letters. 

 The times of excitation were 

 as follows: Curve A, 0.9.; 

 B, 2.4 sec; C, 4. 5 sec; D, 9.2 

 sec; E, 17.5 sec. 



20 



40 



60 80 



Seconds 



100 



120 



adopted it is clear that the decay of phosphorescence involves two distinct 

 processes which merge into one another. In each of these the decay obeys 

 the simple law already referred to, but the rate at which the phosphorescence 

 dies out is greater in the first process than in the second. 



It will be observed that the slant of the curve, for each of the two pro- 

 cesses, is a function of the duration of excitation. 



400 



/vr 



300 



200 



100 



Fig. 48. 



Effect of duration 

 of excitation. 

 Violet light of 

 mercury arc 

 used for exci- 

 tation. The 

 curves were 

 taken in the 

 order by the 

 letter The 

 times of excita- 

 t ion were as 

 follows: Curve 

 A, 27 sec; B, 

 10 sec; C, 3.1 

 sec; D, 1.2 sec. 



20 



40 



60 



80 



100 



120 



140 



160 



Seconds 



In Fig. 48 is shown a series of curves taken under the same conditions as 

 those of Fig. 47, except that the mercury arc was used in excitation instead 

 of the violet end of the ordinary arc spectrum. One is immediately struck 

 by the difference between the two sets of curves. In Fig. 48 there is a 

 strong tendency toward parallelism in the parts of curves corresponding 

 to the second process. In Fig. 47 no such tendency is observable. 



