NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 261 



be diffusely reflected into the collimator of the spectrograph. Photo- 

 graphs which exhibited the selective absorption of the substance were 

 thus obtained. The barium sulphide under investigation showed two 

 narrow absorption bands, indicated below the base line in Fig. i, and 

 a region of general absorption beyond .3 fx. The two narrow bands 

 whose crests as determined from the photographs were at .375 /x 

 and .332 IX obviously correspond with the bands of excitation and 

 sufficiently explain the existence of the latter. 



Similar coincidences between selective absorption and selective 

 excitation were established in the case of the compound SrPbNaF 

 at .355 /A (Lenard's band .358 /x) and of SrZuF at .3601a and .297/^1 

 (Lenard's bands .360/* and .297 /a). The relation is therefore prob- 

 ably a general one, corresponding to that already demonstrated in 

 the case of the selective action of infra-red rays upon phosphores- 

 cence of zinc sulphide, where the maximum effect was found in 

 regions of maximum absorption.^*' 



Spectrophotometric Measurements. 



A detailed spectrophotometric study reveals widely varying de- 

 grees of complexity in the spectra of the different sulphides. Dr. 

 H. L. Howes kindly made for the writer very careful measurements 

 of three characteristic compounds, which may be regarded as pre- 

 liminary to a more extended investigation. 



His method, briefly stated, was as follows : The substance was 

 mounted behind the disk of the synchrono-phosphoroscope and was 

 illuminated by means of the radiation of the zinc spark ; the disk 

 being adjusted so as to afford observation of the phosphorescence in 

 its earliest stages, i. e., after a few ten thousandths of a second from 

 the close of excitation. In place of the photometer used in taking 

 curves of decay a spectrophotometer with two collimators, Lummer- 

 Brodhun cube (L) and constant deviation prism was mounted as 

 shown in Fig. 2. One collimator was directed towards the phos- 

 phorescent surface P, the other towards the comparison light A. 

 The latter consisted of an acetylene flame properly screened. The 

 two slits S, S of the spectrophotometer were of equal width and 



10 Nichols and Merritt, " Studies in Luminescence," Publications of the 

 Carnegie Institution, No. 152, p. 84. 



