64 M. E. Hagenbach's Experiments on Fluorescence. 



Theory of Fluorescence. 



I do not intend here to launch into a criticism of the different 

 theories by which it has been attempted to explain fluorescence. 

 I will merely affirm that none of those theories is adequate to 

 account for the immense variety of the phenomena, and that it 

 will be long before any future theory will be so. 



The ideas expressed by Stokes are still at present the best 

 basis for an attempt at a theory of fluorescence. Thus we must 

 admit with him that the undulations of the aether which strike 

 the fluorescent body set its molecules in motion and cause it to 

 become self-luminous. In this there is a certain analogy with 

 the acoustic phenomenon of bodies vibrating in unison. In one 

 point, however, the difference is very great ; spectral analysis of 

 the light of fluorescence excited by homogeneous light does not 

 give homogeneous light, but an infinity of radiations of different 

 wave-lengths. In this respect fluorescence-spectra approach 

 the spectra of incandescent solids. If the light emanating from 

 a homogeneous substance thus presents an infinite variety of 

 wave-length, this can only be explained, as Stokes has already 

 shown, by the action of forces which are not merely proportional 

 to the first power of the amplitude, and in this way produce 

 undulations for which the period of an oscillation is a function 

 of the amplitude. Undulations of this kind must be admitted 

 in the case of an incandescent solid ; for without that we can- 

 not explain to ourselves the continuity of the spectrum. The 

 incandescence of a solid body constitutes, in my opinion, a 

 relatively simple problem, the solution of which it is indispen- 

 sable to have before we pretend to explain the emission of light 

 by fluorescence. The light emitted by incandescent solids of 

 utterly different natures is, as we know, identical ; it is therefore 

 independent of their intimate molecular constitution, which, on 

 the contrary, has a notable influence in the case of fluorescence, 

 and adds to the complication of the phenomenon. It is clear 

 that, besides the theoretical considerations which Stokes was 

 obliged to use in order to justify the law which bears his name, 

 there are more than one point which it is important equally to 

 take account of — not only the molecular constitution of the 

 substances, but also the more or less great mass of the material 

 molecule compared with that of the ether atom which puts it in 

 motion. It is, besides, only by means of a complete theory of 

 fluorescence that any one will arrive at a satisfactory explanation 

 -of Stokes's law. What has hitherto been attempted of the kind 

 is only a number of more or less venturesome hypotheses. I 

 affirm only that theories deviating from that law would deserve 

 no credence. 



