54: M. A. Kundt on Anomalous Dispersion 



found the same phenomenon in many substances, and proved 

 the relations between anomalies of dispersion and absorption 

 of light and superficial colours, I expressed, in my third com- 

 munication on anomalous dispersion *, t^e conjecture that the 

 gases also, which sometimes possess so energetic an absorption 

 for certain kinds of rays, must exhibit anomalies of dispersion 

 in the vicinity of these rays. I added, however : — " But 

 whether we shall ever succeed in demonstrating the refrac- 

 tion-anomalies in each single absorption-band of the gases and 

 incandescent vapours, some of which show so great a number 

 of thin absorption-lines, must be left undecided." 



I have recently in fact observed in at least one incandescent 

 vapour, that of sodium, a dispersion-anomaly in the vicinity 

 of those rays which this vapour absorbs and emits. What 

 holds good for sodium vapour, will at all events take place 

 with all other absorbing gases and vapours, and, indeed, for 

 the maximum of absorption of each of them. Herewith my 

 previous conjecture is verified by experiment. 



I was led in the following manner to the observation on 

 sodium vapour : — While I, with Dr. Kohlrausch, the Assistant 

 at the Institute, was making, for a lecture, the well-known 

 experiment of the conversion of the bright sodium-line into a 

 dark one, it struck us both that, when the absorbing sodium 

 vapour was very dense and the dark line in the spectrum very 

 broad, its upper and lower margins showed a peculiar round- 

 ing-out in the vicinity of the dark line. On closer examina- 

 tion I soon recognized that we had to do with a dispersion- 

 anomaly, conditioned by the dispersion in the conical sodium- 

 flame. 



The spectrum with the dark line had, on the screen upon 

 which it was thrown, the form shown in the annexed figure. 



w D 



Blue 



The experiment was arranged as follows : — By means of 

 electric light a horizontal intensely bright spectrum was pro- 

 jected, through a prism with the edge vertical, upon the 

 screen. In the path of the rays a Bunsen burner was placed, 

 and with a small iron spoon a piece of sodium introduced into 



* Pogg. Ann. vol. cxliv. p. 132 (1871). 



