196 STUDIES IN LUMINESCENCE. 



secondary effects. 1 But although the exciting light causes a separation of 

 the active molecule into positive and negative parts, it appears unlikely that 

 these parts are the ions of ordinary electrolysis. There are many sub- 

 stances like fluorescein and eosin which fluoresce only when dissociated. In 

 such cases fluorescence can not be due to the recombination of ions; for 

 dissociation and recombination are taking place in an electrolytic solution 

 all the time, and if this were the cause of luminescence we should expect 

 the solution to glow continuously without the action of any exciting light. 

 Fluorescence in such cases must be due to some action upon the ions them- 

 selves. Now it can scarcely be doubted that the absorption of the exciting 

 light is the result of resonance on the part of the molecules or atoms of the 

 active substance; and although the vibrational energy thus imparted to 

 the molecules is rapidly transformed by collisions into translational energy 

 (heat), yet under favorable conditions the molecules might be thrown into 

 such violent oscillation as to be torn apart. It seems, therefore, that the 

 most plausible assumption to make is that the first effect of the exciting 

 light is to produce such violent vibrations as to liberate one or more electrons 

 from the molecule; in other words, to bring about dissociation similar to 

 that produced by Roentgen rays. For the sake of deiiniteness we shall 

 adopt this view in the discussion that follows. 



Among the facts of luminescence that are satisfactorily accounted for by 

 the theory of Wiedemann and Schmidt is the law that the distribution of 

 intensitv and the wave-length of maximum intensity for each band in a 

 luminescence spectrum are independent of the mode of excitation.' 2 Light 

 corresponding to any part of the absorption band may, if sufficiently intense, 

 produce dissociation ; and dissociation may also be brought about by vari- 

 ous other agencies, such as the action of Roentgen rays and kathode rays. 

 But the manner in which recombination occurs, and therefore the form of 

 the luminescence spectrum, will not depend in any way upon the manner in 

 which the dissociation was produced. 



The theory also leads directly to the conclusion that the light emitted 

 during the luminescence of an isotropic substance is unpolarized, whatever 

 may be the condition of polarization of the exciting light, and this con- 

 clusion is in agreement with the experimental results in all cases where 

 polarization tests have been applied. 



The law of vStokes that the light emitted during photo-luminescence is of 

 greater wave-length than the exciting light has always proved a stumbling- 

 block in the development of theories of luminescence, and at first glance 

 the difficulty appears to be as great in the case of the Wiedemann and 

 Schmidt theory as in any of the other theories proposed. Why should the 

 vibrations that occur on recombination differ in period from those origin- 

 ally set up by the exciting light? It is indeed possible that the latter are 

 forced vibrations, whose period bears no simple relation to the natural 

 period of the molecule ; but it is more reasonable to expect that vibrations 

 which bring about actual disintegration are produced by resonance. If 



'It is to be observed that this reasoning does not apply to the case of kathodo-lumiiiescence, since there 

 is no reason why the kathode-ray bombardment should not directly cause chemical changes. The relatively 

 great chemical activity of kathode-ray excitation as compared with excitation by light isprobably connected 

 with this essential difference in the mechanism of excitation in the two cases. 



2 This law, first proposed by Lommel in the case of photo-luminescence, has been tested by the writers 

 for numerous cases of fluorescence excited by light of different wave-length (see Chapter I) and for several 

 cases of excitation by kathode rays and Roentgen rays (see Chapter IX). 



