236 THE ORIGIN OF LIFE 



bility gives rise to fluorescence with the same 

 periods as if the body were exposed simultaneously 

 to the whole radiation of the solar spectrum : which 

 indicates that the spectrum of the emitted rays 

 does not depend upon that of the exciting ones. 

 This point has, as we think, hitherto not been 

 sufficiently emphasised, considering the importance 

 of its bearing on the theory of fluorescence. 



Again, there are many examples in which the 

 spectrum of the fluorescent light of a substance is 

 very similar, if not precisely the same, whether it 

 is excited by light, or by some other means. For 

 instance the spectrum of ruby, amongst a number 

 of other substances, was observed by Sir William 

 Crookes to be the same, whether it is produced 

 by sunlight or by cathode rays.^ These facts in- 

 dicate that the fluorescent molecules are, for the 

 time being at least, independent vibrating systems ; 

 whilst the efi"ect of ultra-violet light, as we shall 

 see, is to bring about the conditions which make 

 it possible for these systems to be formed and 

 then to radiate. 



Since the spectrum of the emitted light is 

 always the same, whatever that of the exciting 

 cause, we have strong evidence in support of the 

 hypothesis that the fluorescence is a secondary 

 phenomenon brought about by the action of the 

 incident light. 



When fluorescence is excited by cathode rays, 



1 Crookes, "Discontinuous Phosphorescent Spectra," Pro- 

 ceedings of the Boyal Society, May, 1881, p. 205. 



