NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 269 



indeed of such bands or systems of great complexity of structure in 

 the case of the uranyl salts. 



The evidence that, in general, the spectra of the phosphorescent 

 sulphides contain more than one band or complex has already been 

 mentioned, c. g., the marked changes of the color of phosphorescence 

 with temperature and during the process of decay, the change of 

 color with the mode of excitation as described by Becquerel, etc. 



In the three sulphides the spectra of which have just been dis- 

 cussed it was thought probable that in spite of the overlapping of 

 the components something might be learned by observing the decay 

 of phosphorescence of different regions of the spectra separately 

 and for this purpose Drs. Howes and Hodge made the following 

 determinations. 



The Decay of Phosphorescence in Different Portions of 



THE Spectrum. 



To obtain the curve of decay for a restricted region of the 

 spectrum the spectrophotometer was used in combination with the 

 synchrono-phosphoroscope and photometer bar as described in a pre- 

 vious paragraph (see Fig. 2). The collimator slits which, to secure 

 the greatest possible detail in the spectrophotometric measurements 

 had been very narrow, were opened to a width of 2.0 mm. so that 

 the brightness of the contrast field would be sufficient to allow the 

 observer to follow the rapidly fading phosphorescence even in the 

 weaker portions of the spectrum. 



The spectrophotometer was set for a selected region and the 

 curve of decay was obtained in the usual manner by observing the 

 position of the comparison lamp upon the photometer bar which 

 gave equality in the contrast field for increasing times after the close 

 of excitation. The range of the readings was from .001 sec. to .03 

 sec. according to the position of the sectored disk upon the shaft of 

 the phosphoroscope. 



In this way a set of curves corresponding to several nearly equi- 

 distant regions within the phosphorescence spectrum was obtained 

 for each of the three sulphides under consideration. 



