LENARD'S RESEARCHES ON PHOSPHORESCENCE 69 



In the same paper he also describes experiments demonstrating 

 that the light total has a limit to which it tends with increasing 

 intensity and duration of excitation ; this limit is independent 

 of the nature of the excitation. 1 When this limit is reached the 

 phosphoroid is said to be fully excited by two different wave- 

 lengths separately, and if a band have two different light totals 

 corresponding to these excitations, these light totals are not 

 added together when the phosphoroid is excited by both wave- 

 lengths at once ; the emission in this case is of the same intensity 

 as that excited by one alone ; this shows that there can only be 

 one kind of centre capable of emitting the particular band 

 which can resonate to both exciting periods. Whilst this is true 

 of the permanent bands, the momentary bands, as Hausser has 

 shown, have no limit of emission intensity ; in this case the 

 intensity increases steadily with the intensity of excitation, and 

 an addition of the two emissions excited by different wave- 

 lengths is effected when these are used simultaneously. 



In comparing the light total caused by excitation by cathode 

 rays and excitation by light, a difficulty arises owing to the 

 fact that the cathode rays cannot penetrate and so excite as 

 thick a layer of the phosphoroid as the light rays : the emitted 

 light increases with the thickness of the phosphorescent sheet 

 used until this is about 1 mm. in the case of excitation by light ; 

 but with the thinnest sheets which can be prepared the cathode 

 rays already excite their maximum of emitted energy. How- 

 ever, the depth of penetration of the cathode rays can be 

 calculated from their known coefficient of absorption : from such 

 calculations Lenard arrives at the conclusion that the total of 

 emitted light is the same whether the exciting agent be light or 

 cathode rays. 



The laws of the decay of intensity of the emitted light were 

 first considered by Becquerel, who, however, investigated the 

 whole of the light emitted from impure phosphoroids and not 

 the separate bands. Since the different bands due to one metal 

 die out at different rates, it is not astonishing that the empirical 

 formula which he proposed represented observation only very 

 roughly. Subsequently Nicholls and Merrit and also Werner 

 put forward as the law of decay of the permanent process the 



formula I = — — — — 2 , in which 1 is the intensity of the light, 

 \C -f- at)" 



1 Which may be light or cathode rays, 



