I4<> STUDIES IN LUMINESCENCE. 



The fluorescence spectrum was also determined for excitation by the 

 ultra-violet rays in the spectrum of the iron spark, and for excitation by 

 Roentgen rays, the results being shown in curves 77/ and IV, respectively. 

 The luminescence excited in both of these rays was weak and the spectrum 

 could be explored only with great difficulty. Curves/// and IV are drawn 

 to a much larger scale than curves /and // in order that they may be shown 

 in the same figure. In reality the intensity at the maximum of curve / 

 is 200 times as great as the corresponding intensity for curve ///, and 

 500 times as great as the maximum for curve IV. It will be observed that 

 the maximum of curve IV occurs at the same wave-length as in the case 

 of kathodo-luminescenee. In the case of photo-luminescence, curve ///, 

 there is an apparent shift toward the shorter waves. We are of the opinion, 

 however, that this shift is not real, but due to errors resulting from the 

 extreme faintness of the spectrum. 



The substance continued to glow for a long time when excited by ultra- 

 violet light, but after excitation by kathode rays the phosphorescence 

 died out with great rapidity. 



WILLEMITE. 



The specimen which was tested was a part of the same piece that had 

 previously been used in determining the decay of phosphorescence in this 

 substance. 1 No change was observable in the specimen, even after prolonged 

 excitation. 



While the kathodo-luminescenee of willemite was not much more intense 

 than that of CdS0 4 , the photo-luminescence, excited as before by the 

 ultra-violet rays from an iron spark, was found to be much brighter than 

 that of CdS0 4 . The luminescence excited by Roentgen rays was, however, 

 only slightly more intense. Curves IV and V, Fig. 146, which show the 

 spectral distribution for the photo-luminescence and the Roentgen lumi- 

 nescence respectively, are plotted to a larger scale than the curves for 

 kathodo-luminescenee on the same figure, as it would otherwise be impossi- 

 ble to see the details of these curves. The maximum intensity for curve / 

 was in reality 120 times as great as that for curve IV and 1,500 times as 

 great as for curve V. 



Inspection of the curves of Fig. 146 seems to justify the conclusion that 

 the luminescence spectrum of willemite has the same form whether the 

 exciting agent is ultra-violet light, Roentgen rays, or kathode rays. The 

 form of the spectrum is also seen to be independent of the potential difference 

 between the terminals of the tube, and is therefore independent of the 

 velocity of the kathode rays. 



When excited by ultra-violet light the willemite used in these experiments 

 showed bright phosphorescence, whose rate of decay was so slow as to be 

 comparable with that of Sidot blende. But when excited by kathode rays 

 the phosphorescence was faint and disappeared within a few seconds. The 

 intense brilliancy of the kathodo-luminescenee during excitation makes this 

 difference especially striking. 



'Nichols and Merritt. Physical Review, XXIII, p. 37, 1906. See also Chapter IV of this memoir. 



