Phosphorescence 363 



vibrates after the exciting light has ceased. Thus, blue light can 

 excite an orange phosphorescence because the natural period of the 

 phosphor vibrators corresponds to orange. 



In the early nineteenth century, theories of phosphorescence be- 

 came more specifically chemical. Ritter (1803), who observed the 

 exciting action of blue light and the quenching action of red light, 

 explained the effects in terms of combustion. The blue light reduces 

 material in the phosphor, thereby allowing subsequent oxidation 

 and phosphorescence to proceed, while red light, by oxidation of 

 the material, quickly suppresses luminescence. Heinrich (1811) 

 looked on light as setting free an acid with which light was bound, 

 while Dessaignes (1809) believed bound water contained the phos- 

 phoric fluid which is essentially electricty. A complex electrical 

 theory was also proposed by Grotthus (1815) , and applied to photo- 

 chemical effects in general. In some ways, he may be regarded as 

 the pioneer in formulating a theory which bears some resemblance 

 to the modern concept of interaction between light quanta and 

 electrons. 



After the publication of G, G. Stokes (1852) and the recognition 

 (see Chapter XI) that certain luminous phenomena in fluorspar 

 and in solutions were not due to light scattering^ but to a lis;ht emis- 

 sion which he called fluorescence, theories of phosphorescence and 

 fluorescence were frequently combined. The vibration theories 

 cam.e back into favor together with the vibratory theory of light 

 itself. It was easier to picture light setting molecules in vibration 

 for a short time than for a long time. As no chemical changes could 

 be observed in a fluorescent body, Stokes himself regarded fluores- 

 cent light as arising from resonance— vibration of molecules, while 

 the continuous character of fluorescent spectra represented damped 

 vibrations as given by a Fourier series analysis. 



W. Eisenlohr (1854, 1856) was one of the first (after Stokes) to 

 present an explanation of the way in which fluorescent light might 

 be excited by interference, comparable to combination tones. In 

 this way ultraviolet light plus another wave length might produce 

 blue fluorescence, and red light plus a second color result in infra- 

 red fluorescence. 



The most important suggestions came from E. Lommel, whose 

 theories of fluorescence in various modifications extended over a 

 considerable period (1862-1895) , with important papers in 1878 

 and 1885. He made use of the laws of vibrating bodies applicable 

 in the theory of sound. According to his latest version, the atoms of 

 an illuminated body were set in vibration, both natural and forced; 

 they were also subject to a damping proportional to their velocity 



