144 STUDIES IN LUMINESCENCE. 



results. For a time the conditions would apparently remain constant and 

 the intensity was found to vary in the same ratio as the current. But 

 some source of disturbance soon developed, so that the next observations 

 were discordant. Some change in the form of tube or the source of current 

 supply will be necessary before this question can be definitely settled. 



Working with the relatively slow kathode rays developed by ultra-violet 

 light, Lenard has found the intensity of luminescence, /, to be related to the 

 discharge potential, V, by the equation 



I = CQ(V-V ) 



where Q is the density of the kathode stream, C a constant, and 1' a mini- 

 mum potential below which no luminescence is produced. 1 The existence 

 of a sharply defined lower limit to the potential that is capable of producing 

 luminescence is called in question by Wehnelt, 2 who used in his experiments 

 a much greater density of the kathode rays. On the other hand a definite 

 lower limit to the velocity has been found by Rutherford 3 in the analogous 

 case of luminescence produced by the a rays of radium. 



Our own experiments with Sidot blende appear at first glance to confirm 

 the conclusions of Lenard, for the voltage-intensity curves, Fig. 148, are 

 straight lines cutting the horizontal axis at a point corresponding to a volt- 

 meter reading of about 1 2 divisions. In the case of the experiments with 

 Sidot blende the current in the tube varied only slightly. We are not 

 justified in concluding, however, that the density of the kathode stream 

 was constant, for the fraction of the current that is carried by the kathode 

 rays is known to vary as the vacuum changes, being practically zero at 

 high pressures and nearly 100 per cent when the pressure is low. In our 

 experiments it was not practicable to determine the amount of this change. 

 It seems scarcely probable, however, that the change was unimportant. 

 If the curves of Fig. 148 could be corrected so as to refer to a constant 

 density of the kathode stream it appears probable that they would differ 

 greatly from the curves plotted. 



CONCLUSIONS. 



In so far as it is possible to generalize on the basis of the small number of 

 experiments here described, the following conclusions appear to be justified : 



1 . In cases of kathodo-luminescence the distribution of intensity through- 

 out each band of the luminescence spectrum is independent of the discharge 

 potential, and therefore independent of the velocity of the kathode rays. 



2. In cases where a band is capable of being excited by light and by 

 Roentgen rays as well as by kathode rays, the form of the band and the 

 position of its maximum are the same for all these modes of excitation. 



We have shown in Chapters III and IV that the distribution of intensity 

 in any given band is independent of the wave-length of the exciting light. 4 

 It appears probable, therefore, that the above conclusion may be stated 

 more broadly, and that the distribution of intensity in each band of a 

 luminescence spectrum is independent of the nature of the exciting agent 

 by which the luminescence is produced. 



Lenard, Annalen der Physik, 12, p. 449, 1903. 



: Wehnelt, Berliner physikalische Gesellschaft, 5, p. 225, 1903. 



3 Rutherford, Radioactivity, second edition, p. 547. 



'Nichols and Merritt, Physical Review, xviii, p. 403; xix, p. 18. 



