LENARD'S RESEARCHES ON PHOSPHORESCENCE 65 



For if similar phosphoroids be prepared with the same active 

 metal, but with different sulphides as bases, we get, passing 

 from one to the other, a series of bands which are in every way 

 analogous to one another, but having maxima which are dis- 

 placed relatively in such a manner that the wave-lengths of the 

 band maximum, divided by the square-root of the specific 

 inductive capacity of the phosphoroid, gives a number which 

 is roughly constant in all the phosphoroids. But this is what 

 theory says would be the case for a Hertzian ^electro-magnetic 

 oscillator vibrating in media of different inductive capacities, so 

 that it is to be inferred that the electron which causes the 

 emission of the light vibrates in and has its period controlled 

 by the nature of the immediate surroundings of the atom to 

 which it belongs. This leads to the assumption that the forces 

 which bind the photo-electric electron to its atom extend out 

 so far into the surroundings of the atom that the mean com- 

 position of these controls its period ; or it may be supposed 

 that the electron moves on the surface of the atom and, in the 

 oscillations which it performs on its return, swings outside the 

 atom while stimulating the emission of light from it— that is to say, 

 from other electrons contained in it. This picture is supported 

 by the results of other experiments on the photo-electric effect. 



Very strong confirmation of this view, which attributes the 

 phosphorescence to the photo-electric action of the light on 

 the atoms of active metal in certain " centres " within the 

 phosphoroid, has been obtained in direct experiment on the 

 photo-electric effect in phosphoroid, performed by Lenard in 

 collaboration with Saeland. As phosphoroids are good insu- 

 lators, as the centres lose negative electricity under the action of 

 light, they acquire a positive charge ; finally, they are raised to 

 such a positive potential that the negative electricity can no 

 longer escape. On calculating from the capacity of the phos- 

 phorescent sheet and the known initial velocity of the photo- 

 electrically liberated electrons, the charge required to raise the 

 phosphoroid to the necessary potential, it is found that, in order 

 that the positive charge actually acquired may be sufficient to 

 stop the escape of electrons, only a fraction of the surface can be 

 charged by it : this is strongly in favour of the theory that there 

 are certain centres which alone take part both in the phos- 

 phorescent and photo-electric action of the phosphoroid. 

 Further, it has been shown by experiment that the two effects 

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