62 



STUDIES IN LUMINESCENCE. 



The well-known effect of the red and infra-red rays in suppressing the 

 long-time phosphorescence of various substances led us to think that these 

 rays might also prove effective as a means of restoring the screen to a 

 standard condition. This conjecture proved to be correct, and exposure 

 to the red and infra-red rays of a 50 c.p. lamp at a distance of about 20 cm. 

 was found to be both convenient and satisfactory. A piece of ruby glass 

 placed in front of the lamp served to remove the more refrangible rays. 

 An exposure of a few seconds to the rays that passed through the ruby glass 

 was sufficient to bring the screen into what seemed to be a definite standard 

 condition. A longer exposure was, however, ordinarily used. 



The changes produced by excitation and the effect of the red and infra- 

 red rays are illustrated by the curves shown in Fig. 50. Curve A shows 

 the behavior of the screen when exposed for 10 seconds after resting in the 

 dark for 24 hours. Curve B is that corresponding to an exposure of 2 

 minutes; curve C was taken with an exposure of 10 seconds immediately 



Fig. 50. Illustrating 

 relative effect of rest 

 and exposure to infra 

 red. 



A, 10 sec. excitation after 

 rest of 24 hours in the 

 dark. B, 2 min. excita- 

 tion. 



C, 10 sec. excitation im- 

 mediately after B. 



D, 10 sec. excitation after 

 exposure of 4 min. to infra- 

 red from 50 candle-power 

 lamp. 



Curves A', B' , C, D' cor- 

 respond to .4, B, C, IK 

 except that / is plotted 

 in place of l~ Js . 



after curve B; and curve D, also with an exposure of 10 seconds, was taken 

 after the screen had been exposed to the red and infra-red rays for 4tninutes. 1 

 In some respects the behavior of the screen is analogous to the magnetic 

 behavior of iron. When iron is magnetized a certain residual magnetization 

 remains after the removal of the magnetizing force, and the effect produced 

 by a subsequent magnetizing force depends upon the magnetic history of 

 the specimen. Similarly some change is produced in Sidot blende by excita- 

 tion which does not immediately disappear upon the removal of the excit- 

 ing light, and which modifies the effect produced by subsequent excitation. 

 The analogy is rendered more striking if this property of Sidot blende is 

 exhibited in a different manner, as in Fig. 51. In the curve plotted in this 

 figure the abscissa of each point shows the duration of excitation, while 

 the ordinate gives the corresponding intensity of phosphorescence after 30 

 seconds decay. The resemblance of the curve to a hysteresis loop for iron 

 is striking. 



Experiments dealing with the effect of the infra-red rays during and after excitation will be found in 

 Chapter V of this memoir. 



