Fluorescence 397 



of physics. The phenomena studied by Herschel and Brewster were 

 reinvestigated in detail, leading Stokes to the conclusion that the 

 light beam in fluorspar was actually emitted, and he spoke of " true 

 internal dispersion " to distinguish it from " false internal disper- 

 sion " or scattering, such as makes the smoke particles in air appear 

 blue. He also used the term " dispersive reflection." In a footnote 

 (p. 479) to the first paper, Stokes wrote " I confess I do not like the 

 term. I am inclined to coin a term and call it fluorescence, from 

 fluor-spar, as the analogous term opalescence is derived from the 

 name of a mineral." In the second paper (p. 387) he definitely re- 

 solved to use the word " fluorescence " and referred to " those cases 

 in which the fluorescent light is yellow." 



Thus " fluorescence " came to be used in connection with the 

 study of both solid (fluorspar) and liquid material, although the 

 duration of light is of the order of seconds in the case of fluorspar 

 and 10"^ second in the case of quinine or fluorescein solutions. 

 Stokes also held that phosphorescence and fluorescence had much 

 in common, in that short waves are the best exciters for both, and 

 that the two phenomena were often found in the same material. 

 For example, fluorspar, which was blue by " internal dispersion," 

 also became blue phosphorescent on heating slightly. The two were 

 not always linked, however, as Iceland spar (calcite) , which was 

 thermophosphorescent, exhibited no fluorescence. 



Another difference pointed out by Stokes had to do with the 

 spread of the excitation as shown by Canton's phosphorus: " When 

 one part of a phosphorus is excited, the phosphorescence is found 

 gradually to extend itself to the neighboring parts. In this respect a 

 substance which exhibits internal dispersion presents a striking con- 

 trast. The finest fixed lines of the spectrum are seen sharply defined, 

 whether in a solution or in a clear solid, or on a washed paper." 



Stokes emphasized two methods of detecting fluorescence. First 

 the method of complementary absorption by which a substance was 

 illuminated through a filter by one kind of light (say blue) and 

 observed through a filter (say yellow) , opaque to the blue light but 

 which might allow any yellow fluorescent light to pass. Second, the 

 method of spectral illumination by which light from a prism illumi- 

 nates the material in such a way that emission of the fluorescent 

 body by any wave-length of exciting light can be observed. 



By the second method Stokes' law was proclaimed, that the fluo- 

 rescent light is of longer wave-length than the exciting light, or that 

 the light is always " degraded " in exciting fluorescence, as Stokes 

 expressed it. One of the most spectacular observations of Stokes 

 came in connection with a test tube of quinine solution, which 



