PHOSPHORESCENCE SPECTRA. 57 



plish this without changing any conditions except the mode of excita- 

 tion the tube containing the crystal under observation was mounted 

 within an unsilvered cylindrical Dewar flask and cooled to the tempera- 

 ture of liquid air. Light from a carbon arc was focussed upon the 

 crystal through the walls of the Dewar flask and of the vacuum-tube, 

 producing intense fluorescence, but there was no after-glow of duration 

 sufficient to be detected. The kathode discharge, however, caused the 

 persistent phosphorescence already described and the effect appeared 

 to be distinctly cumulative, requiring excitation for several seconds. 

 After the phosphorescence had died away, photo-excitation was re- 

 sumed, and this process was repeated many times without observable 

 change in the effect of the light. 



IDENTITY OF THE SPECTRA DURING FLUORESCENCE AND KATHODE- 

 PHOSPHORESCENCE. 



To determine whether the spectrum, during this persistent phos- 

 phorescence, corresponded with the fluorescence spectrum, settings 

 on several of the brightest bands were made with the Hilger spec- 

 troscope. The result was the same as the observations upon the brief 

 phosphorescence following photo-excitation, described in an earlier 

 paragraph of this chapter; i. e., the spectra were found to be identical 

 during and after excitation and remained unchanged in character as 

 long as they were visible. 



CURVES OF DECAY FOR THE KATHODE-PHOSPHORESCENCE. 



Misses Wick and McDowell also determined the law of decay for 

 the three salts (1, 2, and 3) selected for investigation. Since the effect 

 lasted for several minutes, it was possible to use the method commonly 

 employed in such measurements. The arrangement of the apparatus 

 is shown in figure 44. 



FIG. 44. 



A Lummer-Brodhun cube A was placed at one end of a track XY, 

 about 3.5 meters long. The crystal B was placed opposite one face of 

 the cube. The comparison source L was a 5-volt tungsten lamp placed 

 in parallel with a suitable rheostat upon a 55-volt circuit. The lamp 

 was mounted in a carriage C, running on the track XY, on which, at 

 intervals of about 25 cm., stops were placed. Green, blue, and ground 

 glass absorption plates P and P' were inserted to obtain a comparison 

 source of the proper color and intensity. A chronograph was used 

 to record the time. The zero of time was in every instance recorded 

 when the primary circuit of the induction coil was broken. When the 



