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XLYI. Tlie Fluorescence and Absorption Spectra of Sodium 

 Vajjour. Bij Prof. K. W. Wood atid Mr. J. H. Moore*. 



[Plates XIII. & XIV.] 



The Fluorescence of Sodium Vapour. 



'^jPHE green fluorescence which sodium vapour exhibits 

 JL Avhen ilhiminated with an intense beam of white light 

 was first observed and studied by Wiedemann and ^Schmidt 

 (Wied. Ann. Ivii. p. 447, 1896). The method which they 

 employed was to heat the metal in an exhausted glass bulb, 

 concentrating a beam of sunlight on the vapour by means of 

 a lens. A cone of green light is seen where the intense 

 beam enters the mass of metallic vapour. Examined with 

 the spectroscope this light was found to consist of a band in 

 the red, a narrower band nearly in the position of the D 

 lines, and a broad green band, which appeared to be broken 

 up into channels or bands. The wave-lengths of some of 

 these bands were roughly determined, but the authors do not 

 appear to have ascertained exactly the relation which they 

 bore to the absorption-bands which appear in the same part 

 of the spectrum. 



Inasmuch as we have at the present time no very satis- 

 factory theory of fluorescence, and as practically all quanti- 

 tative work has been done with solutions, it seemed worth 

 while to make a careful study of the relation between the 

 fluorescent light emitted by sodium vapour and the light 

 absorbed by the vapour under the same conditions. The 

 chief points of interest which have been brought out by 

 these investigations are the establishment of the fact that the 

 fluorescent spectrum coincides exactly with the absorption 

 spectrum, band for band and line for line, and a determi- 

 nation of the relation existino- between the wave-lenoth of 

 the light which provokes the fluorescence and the nature of 

 the fluorescent spectrum. The somewhat remarkable fact 

 has been ascertained that the D-line absorption is in no way 

 connected with the production of the fluorescence^ though 

 the absorption at this point of the spectrum is much more 

 intense than at any other. 



By means of improved apparatus we have not only been 

 able to photograph the fluorescent spectrum by means of a 

 concave grating, but have been able to observe by means of 

 a grating the fluorescent spectrum when provoked by appro- 

 ximately monochromatic light obtained with the Fuess 

 monochromatic illuminator. The results of the work throw 

 * Communicated by tlie Authors. 



