Superficial Resonance of Sodium Vapour. 1025 



No attempt was made to photograph the spectrum of the 

 resonance radiation excited by D b as it is quite certain that, 

 if the frequency of D 2 does n °t give rise to D 1? the same will 

 hold true for D 1? as in all cases of resonance spectra the 

 wave-lengths longer than those of the exciting light are 

 much more intense than the shorter ones. 



The mechanisms which produce the D 1 lines are not, how- 

 ever, isolated completely, for it has been shown by one of us * 

 that excitation of the vapour in the region of the channelled 

 spectrum by means of blue-green light causes the appearance 

 of the D lines in the emission spectrum, or at least of a yellow 

 band which coincides with the position of the D lines. This 

 band may, however, correspond to a curious band spectrum 

 which is symmetrical about the D lines which appeared in 

 the spectrum excited by the cathode rays. 



Laboratory of M. Bouty, 

 La Sorbonne, Paris. 



OIX. Photometric Investigation of the Superficial Resonance 



of Sodium Vapour. By L. Dunoyer and R. W. Wood f. 



[Plate XVII. figs. 8 & 9.] 



THE vapour of sodium, relatively cold, is capable of re- 

 emitting the D line radiations, when one concentrates 

 on it light containing these same radiations. This was 

 demonstrated by one of us % in 11*05, the image of an oxy- 

 hydrogen sodium flame being formed by a large condenser 

 along the axis of a highly exhausted tube containing a 

 fragment of metallic sodium heated by a small Bunsen 

 flame. At the same time it was shown that the cone of 

 luminosity formed by the exciting rays retreated towards the 

 wall, as the density of the vapour increased until there 

 remained only a thin skin of yellow light, which lined the 

 inner wall o£ the tube. 



The experiment, as carried out at this time, was of short 

 duration, and it did not appear possible to carry on any 

 extensive investigations, with the disposition of the apparatus 

 then employed. 



The method of exciting the resonance has, however,, 

 recently been greatly improved by one of us § by usin<>- 

 small glass bulbs, the walls of which are very carefully freed 

 from occluded gases by prolonged heating, pure sodium beino- 

 introduced into them by distillation. A further improve- 

 ment consists in the employment of a Meker burner fed by 



* Wood, Phil. Mag. x. p. 408 (1905). 



t Communicated by the Authors. 



X Wood, Phil. Mag. x. p. 513 (1905). 



§ Dunoyer, Journal de Physique, iv. p. 17 (1914), 



