70 THE ORIGIN OF LIFE 



types of luminosity we have described ; and that, 

 although perhaps in many instances our theory is far 

 simpler than others, they may help to give us some 

 material with which to test it, when other facts we 

 shall come in contact with may fail by their com- 

 plexity to give us that support in all the entirety 

 that might be wished for. 



In no instance is this theory more approximately 

 true than in the case of fluorescence, which for a long 

 time has been regarded as a simple dynamical 

 phenomenon, which Stokes illustrated by a model 

 consisting of a number of pellets fastened to a string, 

 as we shall have occasion to discuss. This elegant 

 model, although illustrating the change of refrangi- 

 bility, does not give a clue to some of the other facts 

 which accompany or are associated with the phenomena 

 of fluorescence ; and therefore, useful though it is, so 

 far as it goes (in its own way), there can be no 

 question that, whilst explaining roughly the change 

 of frequency, from higher to lower, it yet gives no 

 satisfactory explanation of the true nature of the 

 phenomenon of fluorescence itself. 



For this means not merely a change of refrangi- 

 bility, but also a change of absorption ; the excep- 

 tion to Kirchhoff's law which was assumed to exist 

 in these phenomena being by no means justified by 

 facts. If fluorescence is merely a disturbance which 

 has been aroused in previously existing modes of 

 vibration, it can be shown, without difficulty, that the 

 absorption of light of the same period must be in- 

 dependent of the amplitudes of the vibrations, and 



