PHOSPHORESCENCE SPECTRA. 



39 



When the sectored disk WW is so adjusted on the shaft that the 

 closed sectors conceal the phosphorescent surface during excitation by 

 the spark, an observer, looking through the open sectors as they pass, 

 sees the phosphorescence as it appears a few ten thousandths of a second 

 after. The apparatus is thus suitable for the study of phosphorescence 

 of very short duration or of the earliest stages in cases of slow decay. 

 By shifting the sector on the shaft it is possible without variation in 



o 



, 



FIG. 27. 



FIG. 28. 



the rate of rotation to make observations at the very beginnings of 

 phosphorescence and to compare, by simultaneous vision, the appear- 

 ances just before and immediately after the close of excitation, or, 

 on the other hand, the earlier with the later stages, up to about 0.004 

 second. The photometer, spectroscope, spectrophotometer, camera, 

 etc., may all readily be used with this form of phosphoroscope and 

 studies of the most varied character become possible. 



Phosphorescence is commonly regarded simply as the after-effect of 

 fluorescence, the emission spectrum immediately after the close of exci- 

 tation being identical with that immediately before excitation ceases. 

 This has hitherto been only an assumption, since it is thinkable that 

 the process which prepares a substance for phosphorescence might pro- 

 duce emission during excitation differing from that which consti- 

 tutes phosphorescence and which together with the latter would be 

 present during fluorescence. It is also thinkable, although unlikely, 

 that the phosphorescence might contain some components requiring 

 a measurable time for development and observable only after an appre- 

 ciable interval. 



This is a matter which it would be very difficult to settle in the cases of 

 phosphorescence hitherto studied, because the spectrum of fluorescence 

 and phosphorescence consists of broad bands or complexes of overlap- 

 ping bands, and almost the only criterion of identity is that of color. 



