268 



NICHOLS— EFFECTS OF TEMPERATURE 



[April 22, 



The formation of these free ions is a gradual process ; measured 

 in terms of the time of vibration of hght, indeed, it is ahuost 

 infinitely slow. If the fluorescence of a body subjected to moderate 

 illumination be measured from second to second it will be found to 

 increase in brightness, first rapidly, then, more and more slowly ; 

 approaching a maximum in some cases only after several minutes. 



From such observations a sort of saturation curve may be 

 plotted. In Fig. i, which is from measurements by Professor 

 Merritt and the present writer,^ curve A is such a saturation curve 



5 lO 15 Minutes- 



Fig. I. Curves of saturation and of decay. 



obtained by observing the increase in the brightness of fluorescence 

 of a specimen of Sidot blende during an interval of fifteen minutes. 

 Immediately upon the cessation of excitation the light from the 

 fluorescent body begins to decrease; first rapidly, then more and 

 more slowly and this phase of the phenomenon which may likewise 

 be expressed by a curve, called the " curve of decay " is called phos- 

 phorescence. Curves a, h and c in Fig. i are the curves of decay of 

 the phosphorescence of this specimen of Sidot blende when excita- 

 tions is ended after 15 min., 5 min., and i min. respectively. The 

 law of decay is always the same although the length of time re- 

 quired for the afterglow to become so feeble as to be invisible may 

 vary from many hours'^ to an immeasurably small fraction of a 

 second. 



' Nichols and Merritt, Physical Rcriczv, Vol. XXIII., p. 46. 



° Surfaces coated with the ordinary phosphorescent zinc sulphide or Bal- 

 main's Paint if exposed to sunlight and taken into a dark room have been 

 know to continue to glow for mpnths with sufficient intensity to fog photo- 

 graphic plates. 



