360 History of Luminescence 



emitted light was not polarized. This statement is generally true 

 for isotropic substances. 



The emission of polarized fluorescent or phosphorescent light 

 appears to have been first noted for platinocyanide crystals by Stokes 

 in 1852. It was also studied by J. L. Grailich in 1858 (see Chap. 

 XI) . Later, Professor Maskelyne and Crookes reported polarized 

 fluorescence in the last section of Crookes' (1879) paper on cathode 

 effects. These men observed the polarization of the luminescence of 

 certain precious stones (emerald, sapphire, ruby) exposed to cathode 

 rays, but not of others (diamond, beryl) . 



The most extensive work came later by L. Sohncke (1896) and 

 G. C. Schmidt (1897) . Schmidt made a systematic search for 

 polarized luminescence of matter in various states, finding no 

 polarization in gases or liquids, but its frequent occurrence in aniso- 

 tropic crystals, no matter whether the light emission is classified as 

 a fluorescence, a phosphorescence or a thermoluminescence. An 

 apparent polarization of the cathodoluminescence of glass under 

 pressure was later denied by Schmidt (1899) . 



THE PHOTOELECTRIC EFFECT AND PHOSPHORESCENCE 



Another field of inquiry which cannot be attributed to Becquerel 

 is the relation between phosphorescence and electrical conductivity. 

 After the discovery that ultraviolet light influenced the electrical 

 properties of bodies, affecting their surface charge (H. Hertz, 1887; 

 Hallwachs, 1888) as the result of emission of negative electricity, 

 the phenomenon (called a surface photoelectric effect) was studied 

 by J. Elster and H. Geitel in an extended research. Their first paper 

 (1889) dealt with the fact that alkali metals and Balmain's paint" 

 were sensitive to visible light, and in 1891 they announced that 

 fluorspar and certain other minerals showed the phenomenon. G. C. 

 Schmidt (1897, 1898) studied " Die Beziehung zwischen Fluores- 

 cenz und Actinoelectritat " in solids and liquids. Much later P. 

 Lenard and S. Saeland (1909) showed that the long wave-length 

 limit for the photoemissive effect was also the limit for exciting 

 luminescence. 



That light can affect the electrical conductivity of material like 

 selenium has been known since the publication of Willoughby 

 Smith in 1873,^* but the relation between phosphorescence and 



'3 An impurity phosphor (CaBiS) of the type which exhibits the photoelectric 

 effect. Other luminescent materials such as uranyl salts, platinum double cyanides, 

 or organic compounds have given negative results. 



''^ Jour. Soc. Tel. Eng. 2: 31, 1873. An extensive study was made by W. G. Adams 

 and R. E. Day (Phil. Trans. 167: 328-349, 1876) . 



