260 NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 



enclosed areas BB in the ultra-violet show the regions capable of 

 exciting phosphorescence. These two crests or so-called bands of 

 excitation (Erregungsbande) have fixed positions as to wave-length, 

 for each sulphide. 



Significance of the Bands of Excitation. 



It seemed to the writer probable that these regions of maximum 

 excitation, the positions and appearance of which had long since 

 been beautifully depicted by Becquerel in the work already cited, 

 were due to the presence of absorption bands. Dr. H. E. Howe 

 who was employed during the past summer in the study of the ultra- 

 violet absorption spectra of certain fluorescent solutions, was kind 

 enough to test this hypothesis. Following the method developed by 

 Stokes and by Becquerel and subsequently used by Lenard and 

 Klatt, the phosphorescent substance was exposed to the dispersed 

 rays of a large quartz spectrograph. The source of light was the 

 powerful submerged aluminum spark described by Henri^ and sub- 

 sequently employed by Howe° in his study of absorption spectra. 

 This afit'ords a continuous spectrum of great intensity extending to 

 about .2/x. A considerable portion of the ultra-violet spectrum was 

 found capable of exciting fluorescence. In the case of a barium 

 sulphide with lead with a flux of sodium sulphate this broad band 

 of excitation, corresponding to Lenard's " Momentanband," extended 

 from .42 ja to about .23^1. It is indicated by the dotted line in the 

 lower diagram in Fig. i. Upon this were gradually developed two 

 narrow crests or maxima which glowed for sometime after the close 

 of excitation, the "Dauerbande" of Lenard. The wave-lengths of 

 these crests were estimated as .380 /* and .335 /-i, Lenard gives for a 

 sulphide of similar composition .377 jx and .332 fx respectively, as 

 shown in the upper diagram. 



To obtain the absorption spectrum of these sulphides by trans- 

 mission is impracticable on account of their great opacity, but the 

 following procedure was in some instances successful. A thin layer 

 of the substance was pressed between quartz plates, and mounted in 

 front of the slit in such a position that rays from the spark would 



8 V. Henri, Physikalische Zciischrift, 14, p. 516, 1913. 



3 H. E. Howe, Physical Review, 2, VIII., December, 1916. 



