NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 259 



thousandths of a second after the cessation of excitation as well as 

 later, various marked changes of color during decay not previously 

 noted could be detected. These changes were readily explained by 

 the assumption of overlapping bands, one of which decays with 

 great rapidity and vanishes in a few thousandths of a second, while 

 the other persists. The actual existence of these two components 

 was readily verified : 



1. By observing the spectrum of the light as viewed through the 

 openings of the phosphoroscope. One end of the band could be 

 seen to collapse immediately after the cessation of excitation, i. e., 

 the end towards the violet in the case of the luminous barium 

 sulphides and the end towards the red when the sulphides of calcium 

 or strontium were under observation. 



2. By exciting the substance at the temperature of liquid air. 

 Under these conditions the persistent band was completely destroyed 

 leaving only the band of short duration visible in the phosphoro- 

 scope; with consequent change of color. 



LENARD. 



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HOWE. 



.3|00 ;;4J0 5100 .6|00 



Fig. I. 



It should be noted in this connection that in their original paper^ 

 Lenard and Klatt depicted these spectra as complex, while in his 

 latest paper, already cited, Lenard prefers to regard them as single. 

 This later view may be most briefly and conveniently indicated by 

 the upper part of Fig. i, which is a typical diagram reproduced from 

 Lenard's plate. Here the shaded area represents the location of the 

 band of emission, indicated as a single broad band and the two 



"Lenard and Klatt, Ann. dcr Physik, (4), XV., p. 225, 1904. 



