234 THE ORIGIN OF LIFE 



apparently continuous form. Thus, from these con- 

 siderations and from others based upon the change 

 of absorption, it will appear that the apparently 

 continuous spectrum of fluorescent and phosphor- 

 escent substances is made up of a series of 

 fine bands or lines. This discontinuity, however, 

 does not alter the fact that the spectra of such 

 bodies do on the whole seem to be continuous, in 

 other words, that they range round frequencies for 

 each substance ; as for instance in the bands of 

 maximum and minimum in the fluorescent spectrum 

 of uranium glass. 



Independence of Exciting and Excited Rays. 



It is most remarkable that there does not seem 

 to be any connection between the emitted and the 

 exciting light. ^ By the method of crossed prisms 

 it may be shown without difficulty that the period 

 of the fluorescent light is entirely independent of 

 that of the exciting light or even of its nature ; 

 so much so that the incident rays may be those 

 which are very high up in the ultra-violet, and 

 the spectrum of the fluorescent light be the 

 same as if the exciting light were in the blue or 

 violet. 



This is a most noteworthy fact, as it indicates 

 that the luminous vibrations are not forced. 



The experiment is as follows : — 



When the solar spectrum is thrown upon a 

 1 British Assoc. Beport, Belfast, 1902. 



