Fluorescence 403 



published at Wien and Olmutz (1858) , when he was assistant cura- 

 tor of the Royal Mineral Collection in Vienna. Like Stokes, Grailich 

 noticed that the fluorescent light of certain crystals might be polar- 

 ized, when excited by unpolarized light. He also described, among 

 other things, what he called " Doppelfluorescenz " in certain crys- 

 tals, analogous to but different from dichroism. 



Polarized fluorescence of crystals is particularly noticeable among 

 the platinum double salts. Magnesium platino-cyanide crystals stand- 

 ing upright and traversed by a beam of blue light give an orange 

 yellow fluorescence if examined through a Nicol prism when the 

 polarizing plane of the Nicol is perpendicular to the axis of the 

 prism. The fluorescent light is scarlet when the plane is parallel to 

 the axis. If the exciting light is already horizontally polarized, the 

 fluorescent light is yellow, changing to red as the plane of polariza- 

 tion is rotated through 90°. Under certain conditions the crystals 

 emit unpolarized light and many striking color effects can be 

 demonstrated. 



Later studies in polarized fluorescence were made by E. Lommel 

 (1879), R. A. Millikan (1895), L. Sohncke (1896), and G. C. 

 Schmidt (1897, 1899). That the fluorescence of crystals may be 

 polarized when excited by cathode rays was shown by Professor 

 Maskelyne (1879) in an appendix to W. Crookes' (1879) paper, 

 and also by E. Wiedemann (1880) . 



Schmidt concluded that the fluorescent light of isotropic crystals, 

 pure liquids, or gases is not polarized, but most if not all doubly 

 refracting crystals emit polarized fluorescence. Phosphorescent and 

 thermoluminescent light is also polarized in the case of those bodies 

 which emit polarized fluorescence. 



The light of solutions is never polarized although investigation 

 of the latter gave rise to some controversy. Stokes had pointed out 

 how turbidity in a solution might give rise to an appearance of false 

 fluorescence, owing to polarization of light reflected from fine par- 

 ticles. Failure to recognize this fact and also the polarization which 

 light undergoes at the surface of a liquid, gave rise to claims of 

 polarized fluorescence in solution and considerable discussion by 

 A. Lallemand (1869, 1870) and J. L. Soret (1869). 



In 1862 Eugene von Lommel (1837-1899) , professor of physics 

 at Munich, began his investigations on all aspects of fluorescence, 

 which were to extend over much of the rest of the century. His 

 work may be compared with that of Eilhard Wiedemann (1852- 

 1928) on phosphorescence. An older man, Lommel's papers some- 

 what preceded Wiedemann's but included similar subjects. His 

 theory of fluorescence was discussed over three decades. 



