influence; of red and infra-red rays upon sidot blende. 73 



great an effect as a similar exposure to the stronger rays. But this is not 

 true for shorter exposures. While the ultimate effect of the weaker rays 

 is apparently the same, more time is required to produce the change when 

 the rays are of small intensity ; approximately, at least, the change produced 

 depends upon the product of intensity and duration of exposure. 



The experimental data bearing upon this phase of the subject are so 

 meager as to permit of only the most general conclusions, and additional 

 experiments are much to be desired. So far as they go, however, the results 

 indicate that the condition in which the material is left after the excitation 

 and decay of phosphorescence is an unstable one, due perhaps to some new 

 grouping of the molecules of the phosphorescent material. During rest in 

 the dark accidental disturbances of various kinds may cause the substance 

 to return more or less completely to its normal condition. The effect of 

 rest is therefore uncertain, depending as it does upon the extent to which 

 various obscure and perhaps unrecognized agencies are active ; but certain 

 waves lying chiefly in the infra-red region of the spectrum have a definite 

 and positive effect in restoring the substance to its normal condition. 



Fig. 64. 



Effect on the decay curve of exposure to the infra- 

 red for different times. The intensity of the 

 infra-red rays was here only about j,, of the inten- 

 sity used for the curves of Fig. 63. 



Curve/, 10 seconds exposure after 48 hours rest in 

 the dark; II, 2 minutes exposure; III, 10 seconds 

 exposure after 60 seconds infra-red; IV, loseconds 

 exposure after 15 seconds infra-red. 



20 



40 



60 



GO Sec. I0 



INFLUENCE OF THE LONGER WAVES DURING EXCITATION. 



We have seen that a condition is developed in Sidot blende by excitation 

 which is favorable to the production of strong luminescence by a subsequent 

 excitation. A long excitation is therefore more effective than exposure 

 to equally intense exciting rays for a shorter period ; for the favorable con- 

 dition developed in the early stages of excitation makes the exciting rays 

 that act later more effective. The luminescence of such a substance during 

 excitation i. e., the fluorescence will be relatively weak when excitation 

 first begins and will increase in intensity as the exposure continues, reaching 

 a steady value only after a considerable time. In Sidot blende, with the 

 exciting light used in most of our experiments, 3 or 4 minutes were required 

 to reach a steady value. 1 



The steady condition finally reached is manifestly characterized by 

 equality in the rates of development and decay of the condition favorable 

 to luminescence to which we have just referred. If we think of this favor- 



See the preceding chapter of this memoir. 



