On tin Anomalous Dispersion uf Sodium Vapour. 479 



"Note on the Anomalous Dispersion of Sodium Vapour." By 

 W. H. JULIUS, Professor of Physics in the University of 

 Utrecht. Communicated by C. VERNON BOYS, F.R.S. Re- 

 ceived February 17, Read February 20, 1902. 



The importance of the study of anomalous dispersion in gases, 

 both for testing dispersion theories and for investigating the connec- 

 tion of the subject with solar physics,* has induced Professor R. W. 

 Wood to publish a highly interesting paper on the anomalous disper- 

 sion of sodium vapour, t 



In most of hie experiments, Wood deals with a much denser vapour 

 than I had studied ; this may be the reason that he believes he finds a 

 difference between his views and mine about a point on which, in 

 fact, we do agree. 



On p. 160,f Wood describes the spectrum of electric light that 

 has passed through a tube containing sodium vapour of increasing 

 density, and how in a few seconds the vapour becomes so dense that 

 total absorption of all the light between the D-lines occurs. Then 

 he says : " Julius expresses the opinion that this disappearance of 

 the light between the lines is only a result of the strong dispersion " ; 

 and he is of a different opinion. This, however, proves, I fear, that 

 he really did not read my paper rightly, as he suspected might be 

 the case (p. 169). 



Indeed, speaking of the difference in the result, obtained by 

 Becquerel and by myself in studying the dispersion caused by a 

 sodium flame, I suggested|| that " perhaps Becquerel's flame contained 

 more sodium than mine " ; for I felt sure, that in a sufficiently dense 

 vapour the absorption would extend over a broader region of the 

 spectrum. Some lines further, alluding to the absence, in the 

 spectrum, of the light that is strongly deflected by anomalous dis- 

 persion and thus falls outside the spectroscope, I said : " Here, then, 

 we have a case where the absorption spectrum of a vapour exhibits 

 broad bands not deserving the name of absorption bands " ; and 

 then .... "It would be worth while investigating in how far the 

 anomalous dispersion can have influenced cases, in which broadening 

 or reversal of absorption-lines have been observed." 



So it is quite clear that I did not deny the possibility of getting 



* W. H. Julius, " Solar Phenomena considered in connection with Anomalous 

 Dispersion of Light," ' Koy. Acad. Amsterdam Proc.,' vol. 2, p. 575; ' Astron. 

 Xachr.,' 3672. 



t K. W. Wood, " The Anomalous Dispersion of Sodium Vapour," ' Boy. Soc. f 

 Proc.,' vol. 69, p. 157. 



J Loc. cit. 



Loc. cit., p. 169. 



|| ' Roy. Acad. Amsterdam Proc.,' p. 578. 



