70 SCIENCE PROGRESS 



t the time, and a and c are constants ; a formula of this kind had 

 already been used by Becquerel. This seemed to give a fair 

 representation within the observed limits, the time of observa- 

 tion being about thirty minutes. Recently Lenard and Hausser 

 have attacked the problem in great detail and have shown that, 

 inasmuch as according to the conditions of excitation the decay 

 can take place in different ways, so that under certain conditions 

 curves of decay can be obtained for the same band which cut 

 one another, no law can be given without considerable further 

 discussion of the circumstances preceding the after-glow. 

 This is due to a non-homogeneity of the centres, to be mentioned 

 again shortly. They investigate the behaviour of the separate 

 bands. Their experiments on the effect of the amount of active 

 metal present in a pure phosphoroid show that the total of 

 emitted light per unit volume of the phosphoroid — the reduction 

 to unit volume follows from experiments made on phosphorescent 

 sheets of different thickness — rises first of all proportionally 

 with the increase of metal in the phosphoroid, but then 

 turns and becomes constant, provided that the phosphoroid be 

 fully excited in all cases. The law of decay, and therefore to 

 some extent the light total, depends upon the amount of excita- 

 tion, if this be insufficient to excite the phosphoroid fully : the first 

 falling off in intensity is relatively greater for a brief excitation. 

 These and other observations lead to the assumption of the 

 simultaneous presence of permanent centres of different 

 duration : those of small duration will be quickly excited and 

 will quickly decay, whilst the more durable will have a slow 

 excitation corresponding to their slow falling off. This assump- 

 tion accounts for the observed influence of the duration of the 

 excitation on the law of decay, as in the case of brief excitation 

 a relatively much larger number of quickly decaying centres are 

 excited than by a longer excitation. As regards the amount of 

 metal present, if this be small, only more permanent centres are 

 formed in the phosphoroid ; as it is increased, the number of 

 such centres increases until a stage is reached when all that 

 are possible are formed, and then the less persistent " per- 

 manent " centres are produced. After this, the addition of 

 active metal does not increase the number of permanent 

 centres, as experiment shows. The metal then goes to form 

 "momentary" centres, the intensity of the momentary process 

 being exceedingly small for small metal content. Hirsh has 



