210 History of Luminescence 



ions and X-rays, but finally led to discovery of mathematical for- 

 mulae, which applied to the series of lines observed in the gas spectra 

 (see Chapter VII) . 



FLUORESCENCE 



As explained in previous chapters, the peculiar bichromatic ap- 

 pearance of certain solutions, depending on whether they were ob- 

 served from the side or by transmitted light, has been known since 

 the description of an extract of " lignum nephriticum " by Kircher 

 in 1646. There continued to be considerable interest in the phe- 

 nomenon during the late seventeenth century, a period noted for 

 inquiry into the nature of colors, but during the eighteenth century 

 almost no research was carried out except for an occasional descrip- 

 tion of new liquids with the peculiar property of " lignum nephri- 

 ticum " extract. 



At the beginning of the nineteenth century, interest again revived, 

 chiefly because a number of crystalline minerals, for example fluor- 

 spar, were found to behave as did the solutions. David Brewster 



(1838, 1846, 1848) and John Herschel (1845) both attempted to 

 explain the color of a beam of light passing through a crystal or a 

 liquid by " scattering " or by what they called " epibolic dispersion " 

 or " internal dispersion." However, their interpretation was incor- 

 rect, and it remained for G. G. Stokes (1852) to characterize the 

 light as a true emission, actually a phosphorescence of very short 

 duration. At first Stokes spoke of " true internal dispersion " in 

 contrast with scattering or " false internal dispersion," then used the 

 word " dispersive reflection," and finally hit on the term " fluores- 

 cence," from fluorspar, analogous to the term " opalescence," used 

 to describe the play of colors in another mineral, the opal. During 

 the study, Stokes noticed that in producing fluorescence, light is 

 always " degraded," i. e., the shorter wave-lengths excite the fluores- 

 cent emission of longer wave-lengths. This characteristic, now 

 known as Stokes' law, is one of the important generalizations of 

 luminescent study during the nineteenth century. 



Stokes' papers started a wave of investigations on fluorescence. 

 During the last half of the century, many workers studied fluores- 

 cence of all states of matter— gases, liquids and solids— especially in 

 relation to Stokes' law, and attempted to classify the various types 

 observed. Spectroscopic and phosphoroscopic observations were the 

 order of the day. The best known worker in the field was E. Lommel 



(1837-1899) , whose research on luminescence extended from 1862 

 to 1895. A generalization (1875) often associated with his name is 

 the pronouncement that a body only fluoresces by virtue of those 



