142 



STUDIES IN LUMINESCENCE. 



through the spectrum a second time with practically identical results. 

 The points marked by circles in Fig. 147 were determined first, while 

 those marked with crosses were taken afterwards, with no change in the 

 conditions, as a check. It is clear that the spectrum contains two over- 

 lapping bands, and the estimated curves for the separate bands are indicated 

 by the dotted lines. 



Our more recent measurements with Sidot blende were made by the 

 same method as that used with CdS0 4 and willemite. The results are 

 shown in Fig. 148. 



When subjected to the relatively intense kathode rays used in these ex- 

 periments the Sidot blende was found to undergo a rapid change, which was 

 manifested both by a discoloration of the surface and by a diminution in the 



intensity of the luminescence. 

 In Fig. 148, curve III corre- 

 sponds to the same voltmeter 

 reading as curve I, but was 

 taken after the substance had 

 been excited continuously for 

 2 or 3 hours. Not only is the 

 intensity much diminished by 

 prolonged excitation, but the 

 whole character of the lumines- 

 cence spectrum is altered. 



The band at about 0.5 /j, 

 which appears in Fig. 147 and 

 in curves I and 77 of Fig. 148, 

 appears to be the same as that 

 excited by light and by Roent- 

 gen rays. 1 The band at about 

 0.455 M> is a l so excited by 

 Roentgen rays. 2 But this band 

 is either not present in the 

 spectrum excited by ultra- 

 violet light, or is masked by 

 the other bands present in 

 that spectrum. The lumines- 

 cence of Sidot blende, as we have previously pointed out, is extremely com- 

 plex, and the green band at about 0.51^ is the only one which can be 

 readily isolated. 



When excited by kathode rays the phosphorescence of Sidot blende is 

 faint and of relatively short duration. 



The infra-red rays obtained by interposing a thin sheet of hard rubber 

 in the path of the rays from an arc diminished the kathodo-luminescence 

 of Sidot blende only slightly. A long series of settings, alternately with 

 and without infra-red rays, was necessary to make certain that any effect 

 was produced at all. Infra-red rays of the same intensity would have cut 

 down^the fluorescence excited by light to one-half its normal intensity. 3 



'Nichols and Merritt, Physical Review, xxi, p. 247, 1905, Figs. 30, 31, and 35. See also Chapter III, Fig. 40. 



: See Fig. 41, p. 44. 



'Physical Review, XXV, p. 362, 1907. See also Chapter V of this memoir. 



S9/t .60 



Fig. 147. Sidot blende. 



