DECAY OF PHOSPHORESCENCE IN SIDOT BEENDE. 



6l 



We were at first inclined to ascribe this difference to the fact that different 

 kinds of exciting rays had been used in the two cases; but the two sets of 

 curves differ in another respect, namely, in the order in which the curves 

 were taken. In the first case the curves of short excitation were taken 

 first, while in the latter case the curves corresponding to long excitations 

 were the first observed. It appeared possible that the difference in the 

 form of the curves was due to this difference in sequence rather than to the 

 difference in exciting light. 



To test this matter the observations plotted in Fig. 49 were made. It 

 will be noticed that the dotted curves are similar to those in Fig. 48, while 

 the full curves are similar to those of Fig. 47. The mercury arc was used 

 in excitation in all cases. But curves taken after the screen had been sub- 

 jected to the long excitations corresponding to curves C and D differ widely 

 from the curves taken with approximately the same excitation previous 

 to C and D. A comparison of curves A and E illustrates this point, the 

 the duration of exposure being exactly the same in each of these two cases. 



Fig- 49- 



Effect of duration of 

 excitation. Mercury 

 arc. The curves were 

 taken in the order in- 

 dicated by the letters. 

 The times of excita- 

 tion were as follows: 

 Curve .4, 4.3 sec; B, 

 8.2 sec; C, 16.0 sec; 

 D, 56.0 sec; E, 4.3 

 sec; F, 1.4 sec; G, 

 1 .0 sec. 



These results indicate that some change is produced in the phosphorescent 

 material by the action of the exciting light, and that this changed condition 

 persists for a considerable period after all visible phosphorescence has 

 ceased. In other words, the effect of a given excitation in producing phos- 

 phorescence depends upon the previous history of the phosphorescent 

 substance. 



If the screen is allowed to rest in the dark for a number of hours this 

 semi-permanent effect of exposure in part dies out. But rest alone does 

 not restore the screen completely, even if continued for several days. The 

 effect of rest was also found to be somewhat uncertain, being much greater 

 on some occasions than on others. 



Several methods of restoring the screen to a standard condition were 

 tried. Heating the screen to the temperature of boiling water for several 

 minutes and then cooling it again to the temperature of the room seemed 

 effective. But this method required considerable time and has not been 

 thoroughly tested. Cooling the screen to the temperature of liquid air and 

 afterwards warming it gradually to the original temperature seemed to be 

 without effect. 



