44 



STUDIES IN LUMINESCENCE. 



We are of the opinion that the difference is due to the fact that the violet 

 luminescence does not consist of one band only, but of at least two, which 

 are excited by Roentgen rays and ultra-violet light in different relative 

 intensity. The matter will be made clear by reference to Fig. 41. Curve I 

 shows the luminescence spectrum when ultra-violet rays from the iron spark 

 were used for excitation. The rays used were those giving most intense 

 luminescence, and did not lie much beyond the edge of the visible spectrum. 

 The shape of this curve suggests that it is made up of two overlapping 

 bands, one having a maximum not far from 0.48 ;u, while the other has its 

 maximum near 0.42 ju. In curve II the exciting light was at the extreme 

 ultra end of the iron spectrum, and in curve 77/ the magnesium lines near 

 0.33 ix were used in excitation. In these curves also there is every indication 

 that the spectrum consists of at least two bands. 



Unfortunately we have not been able to separate the bands more 



completely. It is to be 

 remembered that the re- 

 solving power of a spec- 

 trophotometer is at the 

 best small, and that in 

 experiments of the kind 

 here described the small 

 intensity of the light 

 studied prohibits the use 

 of a narrow slit. Lumin- 

 escence bands may some- 

 times be separated by a 

 proper choice of the excit- 

 ing light. In the case 

 of Sidot blende the green 

 band is very readily ob- 

 tained alone by using the 

 carbon bands of the arc 

 for excitation. But the 

 two violet bands in the 

 luminescence of this sub- 

 stance are so close to- 

 gether that their regions of excitation appear to overlap throughout nearly 

 their whole extent. The most that we were able to do was to show that 

 ultra-violet rays of different wave-length produced different relative inten- 

 sities in the two bands, as illustrated by curves I and III of Fig 41. 



The broken line, curve IV of Fig. 41, shows the luminescence spectrum 

 excited by Roentgen rays. This is the same curve that appears in Fig. 38, 

 and is introduced in Fig. 41 to facilitate the comparison of the effects of 

 the ultra-violet light and Roentgen rays. It will be observed that each of 

 the curves in Fig. 41 might well result from the superposition of three bands, 

 whose maxima lie approximately ato.42^, 0.48^ and 0.51 p.. In the Roent- 

 gen-ray luminescence the green band is relatively stronger than in the case 

 of excitation by ultra-violet light, while the band at 0.48 /x appears to be 

 entirely absent. In the luminescence produced by the magnesium spark 



0.4-6 



0.50 



O. 



54/i 



Fig. 41. 



Fluorescence spectrum of Sidot blende when excited by the ultra- 

 violet rays of the iron spark (curves / and //), by the ultra- 

 violet of the magnesium spark (curve ///), and by the Roentgen 

 rays (curve IV). 



