THE THEORY OF WIEDEMANN AND SCHMIDT. 1 97 



this is true we have to do with the natural period of the molecule in both 

 cases, and it would seem that the light emitted during luminescence should 

 have the same wave-length as the exciting light. In the discussion that 

 follows we venture to make a suggestion which offers an explanation of this 

 difficulty. 



In the case of photo-luminescence in solids and liquids the active mole- 

 cule is always closely surrounded by other molecules. In general these 

 surrounding molecules belong to the solid or liquid solvent in which the 

 active material is dissolved. If we have to deal with the luminescence of a 

 pure substance (if such cases occur), the surrounding molecules are of the 

 same kind as those which participate in the luminescence phenomena. 

 But in either case the period of vibration will be different from what it 

 would be if the vibrating molecules were isolated. Since the change in 

 period will be relatively great for those molecules that are close to their 

 neighbors and smaller for those that are farther away, it is seen that the 

 natural period will vary through a considerable range as the active mole- 

 cules move about, and at each instant there will exist in the substance 

 molecules having all periods lying between certain rather wide limits. The 

 absorption spectrum of the substance therefore consists of bands rather 

 than of lines. 1 



When a molecule is dissociated by the action of the exciting light, the 

 two parts, being electrically charged, will be more strongly attracted by 

 the molecules of the solvent than was the original neutral molecule. Recom- 

 bination of the separated ions is therefore more likely to occur when the 

 latter are in the immediate neighborhood of the molecules of the solvent, 

 i. e., under conditions which make the period of the resulting vibrations 

 longer, on the whole, than the period natural to the active molecules before 

 dissociation. Since recombination can occur under a variety of conditions 

 a wide range of wave-lengths will be represented in the luminescence 

 spectrum ; the latter also will consist of bands rather than of lines. But 

 on the whole the wave-length of the light emitted during luminescence will 

 be longer than that of the exciting light. The same sort of thing will occur, 

 although in less marked degree, even when the active molecule is itself 

 electrically charged, as in the case of fluorescein. In this case, as well as 

 in that first considered, it may even be that each of the two parts into 

 which the active molecule is dissociated becomes a nucleus to which neutral 

 molecules are attracted, so that the ions become heavy aggregations of 

 molecules. This is the assumption usually made regarding the production 

 of ions in gases by the action of Roentgen rays, kathode rays, etc. If this is 

 the real condition of affairs, the reasons for increased wave-length in the 

 light emitted are still more evident. 



It will be seen that this way of looking at the phenomena of photo-lumines- 

 cence gives what might be called a mechanical explanation of Stokes's law. 

 It does not lead us to expect, however, that Stokes's law will always be 

 exactly followed. The luminescence spectrum and the absorption spec- 

 trum may overlap ; in fact, it is to be anticipated that they will do so in the 

 majority of cases. This is in agreement with the more recent experiments 

 on this subject. 



Essentially this explanation of the broadening of spectral lines has been discussed in some detail by 

 Galitzin, Wied. Ann., 56, p. 78. 1895. 



