7 6 



STUDIES IN LUMINESCENCE. 



determine the ratio of the effects produced by strong and weak infra-red, 

 in spite of the uncertainty in the value of the reflected light. Thus the 

 difference between the ordinate of curve I and the corresponding ordinate 

 of curve II is a measure of the effect produced by weak infra-red ; reflected 

 light, since it affects both measurements, is eliminated by taking their differ- 

 ence. Similarly the difference between the ordinates of curves / and III 

 measures the effect of the stronger infra-red. It is interesting to note that 

 the ratio of these effects is nearly constant throughout the green band. 

 Beginning at 0.562 /* and running toward the violet the ratio has the values : 

 1.32, 1.36, 1.3 1, 1.22, 1.94. Except for the last point, which is so near the 

 edge of the band that the intensity of fluorescence is small, the values are 

 constant to within observational errors. This fact adds another to the 



many that have been observed in the course of our 

 work on luminescence to indicate that each band 

 in a luminescence spectrum behaves as a unit 

 that whatever affects one part of the band affects 

 all other parts of the band in the same proportion. 

 A more satisfactory method of studying the 

 effect in question is to use only ultra-violet light 

 in excitation. All troubles due to reflected light 

 are in this case removed. The results of this pro- 

 cedure in the case of the original Sidot blende used 

 in our earlier experiments are shown in Fig. 67. 

 An iron spark was used as an exciting source, a 

 spectrum being formed by a quartz train and only 

 the ultra-violet rays used. The source of infra- 

 red was an arc lantern whose rays passed through 

 a sheet of hard rubber 0.2 mm. thick. The effect 

 of exposure to longer rays during excitation by the 

 ultra-violet rays of the iron spark was to change 

 the fluorescence from curve I to curve 77. The 

 diminution in intensity brought about by exposure 

 to the longer rays, expressed as a fraction of the 

 ordinary fluorescence at the same wave-length, is 

 given in curve III. 

 More extended experiments are required to determine whether the change 

 in the effect from 38 per cent at 0.546 fx to 20 per cent at 0.480 ju is real or 

 the result of errors. It does not seem likely, however, that experimental 

 errors alone can account for so great a change. In interpreting the results 

 we must bear in mind the fact that the spectrum shown in Fig. 67 obviously 

 consists of two overlapping bands, and that the infra-red effect may differ 

 for the two. It is highly probable also that still another band is present 

 at 0.49 n, as was found to be the case with ultra-violet excitation in the 

 experiments plotted in Fig. 65; and for this band the effect of the longer 

 waves may be different still. It is clear that further experiments on this 

 branch of the subject are needed. 



EFFECT OF THE LONGER WAVES DURING DECAY. 



In studying the effect of the infra-red rays upon the decay of phosphores- 

 cence two methods were used. In the first of these the intensity- of the 



Effect of infra-red upon fluores- 

 cence of "Emanations- 

 pulver" excited by ultra- 

 violet rays of an iron spark. 



