262 



NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 



measurements were made by moving the flame along a photometer 

 bar B^ B in the prolonged axis of the collimator. Settings were 

 made at intervals of 50 Angstrom units throughout the spectrum. 

 On account of the very great range of intensities within the phos- 

 phorescence spectrum it was necessary to increase the effective range 

 of the photometer bar by the interposition of screens for which the 

 reduction factors had been carefully determined. 



Fig. 2. 



The first substance studied in this manner was a strontium sul- 

 phide, with bismuth as the active metal, designated as L. and K. No. 

 13. The spectrum curve obtained by Dr. Howes, using the method 

 described above, is shown in Fig. 3. The complexity of the band 

 is very obvious, there being subordinate crests on either side of the 

 principal maximum. 



The curve suggests at once a group of overlapping bands, so 

 nearly merged that to the eye it would appear as a single simple 

 band. There is moreover a distinct suggestion of a systematic 

 relation. 



