I9I0.] ON PHOSPHORESCENCE AND FLUORESCENCE. 



279 



the light of the mercury arc, give a curve of the form shown in 

 Fig. 15. There is a narrow band with a sharp peak at wave-length. 

 .4g4fi and a group of two or more much weaker, over-lapping bands 

 towards the violet. At the temperature of liquid air the band 

 towards the red is reduced in brightness and is shifted towards the 

 violet, the group of bands of shorter wave-length, however, are 

 greatly increased in intensity and the curve shows the presence of at 

 least four crests, at .480/*, 474,0,, .468/^ and .463/-1. 



'T 



Fig. 14. 



•^y^ 



Fig. 15. 



■fro/*. 



When viewed in the ordinary way with the spectroscope this 

 entire group appears as a single broad band and indeed it has been 

 so designated by Lenard^^ in a recent paper. When however we 

 consider that each of its overlapping components is independently 

 affected, by temperature as to wave-length, intensity and rate of 

 decay it will be seen that any complete and quantitative study of 

 the fluorescence and phosphorescence is a very complicated and diffi- 

 cult matter. 



Numerous spectrophotometric measurements, of the spectra of 

 " Lenard, Ann. dcr Physik, IV., Vol. 31, p. 641, 1910. 



