INDEXED, 



THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



NOVEMBER 1905. 

 



LVII1. The Fluorescence of Sodium Vapour and the Reso- 

 nance Radiation of Electrons. By R. W. Wood, Professor 

 of Experimented Physics, Johns Hopkins University^. 



[Plate VII] 



IN the modern theories of absorption we find frequent use 

 made of the conception of a re-emission of radiant energy 

 by an electron which is vibrating in unison with the incident 

 light-waves. The emitted energy should, however, be of the . 

 same wave-length as that of the exciting waves, and while 

 we have plenty of examples of photoluminescence, it seems 

 probable that in these cases ihe phenomenon is extremely 

 complicated, for the emitted radiation consists of a hetero- 

 geneous mass of waves, usually of slower period than that of 

 the incident light. Repeated efforts have been made by 

 various investigators to detect a - lateral emission of yellow 

 light by sodium vapour when in the act of absorbing sodium 

 light. These efforts have been unsuccessful for reasons which 

 will be presently given. The phenomenon has at last been 

 observed : a dense mass of non-luminous sodium vapour 

 radiating a brilliant yellow light when illuminated by the 

 light from a very intense sodium flame. The same pheno- 

 menon has been observed in the region of the channelled 

 absorption ; in this case, however, radiations of other wave- 

 lengths are emitted, as well as those of the same period as 

 that of the exciting light. This seems to be the first case 

 found of the phenomenon, which it may perhaps be well to 



* Communicated by the Physical Society : read June 30, 1905. 

 Phil. Mag. S. 6. Vol. 10. No. 59. Nov. 1905. 2 N 



