404 History of Luminescence 



Although Stokes' law certainly implies that absorption of light is 

 necessary for emission, Lommel (1875, 1876) emphasized that to 

 excite fluorescence, light must be absorbed, just as in the case of 

 photochemical effects the Grotthus-Draper law declares that light 

 must be to effect a chemical change. Lommel made his observation 

 abundantly clear, that " every dark band in the absorption spectrum 

 corresponds to a bright band in the fluoresceing spectrum of a fluo- 

 rescent solution. . . . The general proposition can therefore be laid 

 down that a body capable of exhibiting fluorescence, fluoresces by 

 virtue of those rays which it absorbs." He applied the statement to 

 phosphors also: " Phosphorescence, like fluorescence, is an effect of 

 absorbed light." The generalization might be called Lommel's law. 

 It came from his study proving empirically that the amount of fluo- 

 rescent light from unit volume of solution is proportional to the 

 amount of absorbed light. In the next century, measurements of 

 energy in the light absorbed and in the fluorescent light were to 

 lead to such important values as fluorescence efficiency and quantum 

 yield. 



Perhaps greater attention in the post-Stokes period was paid to 

 spectral characteristics of the exciting and of the fluorescent light, 

 than to any other aspect of fluorescence. Lommel was only one of 

 the investigators. Stokes' law had not only stimulated much careful 

 study but also aroused considerable controversy. One of the prin- 

 cipal investigators was E. Hagenbach, who undertook a study of 

 chlorophyll fluorescence in 1869. In 1872 his monumental paper, 

 " Versuche uber Fluorescens " appeared in Poggendorff's Annalen 

 der Physik. This and later work included a special study of the 

 spectra of absorbed and emitted light in relation to Stokes' law. 

 The subject also occupied the interests of E. Lommel (1862-1895) , 

 V. Pierre (1862-1866), J. Obermann (1871), O. Lubarsch (1874- 

 1881), H. C. Sorby (1875), J. Brauner (1877), A. Wullner (1878, 

 1883, 1899) , S. Lamansky (1879) , E. Becquerel (1879) , E. Linhardt 

 (1882) , E. Ketteler (1882) , K. Wesendonck (1884, 1897) , F. Stenger 

 (1886, 1888) B. Walter (1888, 1892), W. Bohlendorff (1891), G. 

 Salet (1892) , G. Jaumann (1894) , E. Buckingham (1894) , O. Kno- 

 blanch (1895) , B. Galitzin (1895) , G. C. Schmidt (1896) , J. Burke 

 (1897, 1898) , and others in the next century. The list is a roster of 

 those who have studied fluorescence, although the details of experi- 

 ment and theory are so great that results cannot be presented. Suffice 

 it to say that Stokes' law has been subjected to rigid tests and found 

 to be generally true for both fluorescence and phosphorescence, one 

 of the basic laws of luminescence. A few anti-Stokes emissions have 

 been logically explained by modern theory. 



