Page:Proceedings of the Royal Society of London Vol 69.djvu/499

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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

Dispersion of Light," ' Koy. Acad. Amsterdam Proc.,' vol. 2, p. 575; ' Astron. Xachr.,' 3672.
 * W. H. Julius, " Solar Phenomena considered in connection with Anomalous

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.