The Anomalous Dispersion of Sodium Vapour. 157 



" The Anomalous Dispersion of Sodium Vapour." By R W. 

 WOOD, Professor of Physics in the University of Wisconsin. 

 Communicated by C. V. BOYS, F.I1.S. Received June 10, 

 Eead June 20, 1901. 



[PLATES 1-3.] 



Experimental proof of the dispersion formulae of Sellmeier, Helm- 

 holtz, and Ketteler has for the most part been based upon observations 

 made upon substances in the solid state or in the state of solution, 

 where the range of absorption extends over a considerable portion of 

 the spectrum, and the molecular condition is in all probability vastly 

 more complicated than in the gaseous state. 



In liquid oxygen and in the salts of erbium and didymium we have 

 the only examples, so far as I know, of substances other than gases 

 which exhibit narrow absorption bands, and in these cases the absorp- 

 tion is scarcely strong enough to influence the dispersion to any 

 marked degree. A careful study of the dispersion of some substance 

 with absorption bands as narrow and strong as those of sodium vapour 

 would be of great value in aiding us to pass judgment on the various 

 modifications of Sellmeier's original theory. 



The anomalous dispersion of sodium vapour in the immediate 

 vicinity of the D lines was first observed by Kuridt, and has since 

 been studied by Becquerel and Julius, but none of these investigators 

 has traced the effect over a range of the spectrum greater than two or 

 three times the distance between the lines. 



In a previous paper* I have shown that a spectrum closely resem- 

 bling the bright-line spectrum of incandescent sodium can be pro- 

 duced under certain conditions by the anomalous refraction of white 

 light in a non-homogeneous atmosphere of sodium vapour, in precisely 

 the manner in which Julius very ingeniously suggests that it may 

 occur in the reversing layer of the sun, giving rise to the phenomenon 

 of the " flash spectrum." In view of the seeming importance of the 

 subject in its bearing on the subject of the dynamics of dispersion, 

 and its possible connection with solar-physics, I have undertaken to 

 determine whether sufficiently accurate quantitative data can be 

 obtained, to make a test of the dispersion formula, as applied to 

 sodium vapour, possible. 



Though the absolute values of the refractive indices which have 

 been found are not as satisfactory as is desirable, very interesting 

 relative values have been obtained, and >vhat is of especial interest, 

 the dispersion has been traced and measured throughout the entire 

 range of the visible spectrum, the refractive index for all waves of 



* 'PhU. Mag.,' May, 1901. 



