52 STUDIES IN LUMINESCENCE. 



several different samples of which were tested. Henry states that in the 

 case of one specimen the results were represented by an exponential law 

 (eq. i) for 14 seconds, while other preparations obeyed the law 



,-0.5936^ + 27-l8 ) = l6475 (3) 



Henry appears to have been convinced of the correctness of the latter law 

 and proposed a new t)^pe of photometer which made use of the gradually 

 decaying phosphorescence of Sidot blende in the measurement of faint 

 sources of light. 1 While the fact that the constant in eq. (3) is given to 

 five significant figures indicates an accuracy that is unusual in photometric 

 measurements, the lack of experimental data in the paper referred to 

 makes it difficult to form an independent opinion of the significance of the 

 conclusions. 



The decay of phosphorescence was considered from the theoretical stand- 

 point by H. Becquerel 2 in 1891. Upon the assumption that the light 

 emitted during phosphorescence was due to molecular vibrations set up 

 by the action of the exciting light and afterwards gradually dying out, it 

 was shown that the law of decay would be determined by the nature of the 

 damping forces. If the vibrations meet with an opposing force propor- 

 tioned to the speed it was shown that an exponential law of decay would 

 result ; while if the resistance is proportional to the square of the speed the 

 law of decay would take the form 



7 =(a-r-V (4) 



It will be noticed that the empirical law proposed by E. Becquerel reduces 

 to (4) in case ^ = 0.5. 



In the case of a substance whose phosphorescence spectrum contains 

 several bands Becquerel proposed the expression 



in which there is one term in the summation for each band. Upon testing 

 this law with the data obtained by E. Becquerel for a calcium sulphide 

 giving blue phosphorescence it was found that the results could be expressed 

 by the use of two terms in the above series with great accuracy. The 

 existence in the spectrum of this substance of two bands possessing inde- 

 pendent properties could be demonstrated in various ways. 



In the derivation of the law proposed by H. Becquerel the assumption 

 is that the vibrations set up by the action of the exciting light continue 

 during several minutes or even hours. This would imply either that the 

 vibrating atoms or molecules exist during this time without collisions with 

 other molecules, or else that such collisions are without effect upon the 

 vibrations. Neither of the suppositions seems to us tenable. But the 

 law nevertheless appears to be of very general application. We shall 

 show later that the same law may be derived from entirely different theo- 

 retical considerations. (See Chapter XV.) 



"Comptes Rendus, 115, p. 602. 

 J Comptes Rendus, 113, p. 618, 1891. 



