LENARD'S RESEARCHES ON PHOSPHORESCENCE 71 



shown that for a large number of bands the intensity of the 

 momentary process continually increases with the amount of 

 active metal, whilst, as stated, the number of permanent centres 

 soon reaches a limit. He has also shown that a higher 

 temperature is needed to prepare phosphoroids of pronounced 

 after-glow, which falls in with the hypothesis, as other considera- 

 tions show that the centres of long duration must be very large 

 atomic complexes which would take some time to form, the 

 production of which would accordingly be much facilitated by 

 the increased diffusion consequent on a higher temperature in 

 the preparation. Short heating at comparatively low tem- 

 perature will give rise to a phosphoroid which shows a 

 good momentary process and only a very faint permanent 

 process. 



Some account has now been given of the work carried out 

 on the phosphorescence of pure phosphoroids of known com- 

 position which seem to offer by far the best opportunity of 

 obtaining a true — or perhaps one should say useful * — insight 

 into the mechanism of phosphorescence. The information so 

 obtained is particularly helpful in the study of the emission of 

 light in general, as we are dealing with single widely separated 

 centres of emission provided by the atoms of the active metal. 

 A great amount of interesting work has been done on the 

 phosphorescence of substances of doubtful composition, especi- 

 ally for excitation with fast cathode rays, which excite a short 

 phosphorescence in nearly all substances. It is hard to give 

 a condensed account of this work, consisting, as it largely 

 does, of observations under imperfectly known conditions of very 

 complex phenomena : the lack of any broad theoretical basis 

 for the class of experiments referred to renders generalisation 

 as to many very interesting but apparently independent 

 phenomena which have been observed almost impossible. I 

 have therefore and from considerations of space confined myself 

 to the long series of connected experiments made by Lenard 

 and his collaborators and to the other experiments known to 

 me which bear directly on the questions under discussion. 

 May this brief description of systematic labours and able 

 theorising help to demonstrate the significant and far-reaching 

 results to which the careful study of a single, apparently 

 insignificant, phenomenon may lead. 



1 A distinction without a difference, according to the pragmatists, 



