( 324 ) 



Dispersion l)an(ls alwavs appear in tlie proximity of al)sorption 

 lines, covering- tliem more or less symmetrieallv : lliey show great 

 varietv in width and strength, and the distribution of the light in 

 them may be irregular, so as to give the impression that one is 

 witnessing cases of shifting or doubling or complicate reversal phe- 

 nomena of widened abs()rj)tion lines. All these cases can be produced 

 almost at ])leasure in the absorption sj)ectrum of sodium vapour by 

 merelv varving the structure of the non-homogeneous medium through 

 which the light is made to ti-avel. 



In the spectrum of the various parts of the solar image dispersion 

 bands play an important part ^). We can scarcely doidtt that they 

 are also preseni in steUai' specli-a; for the light coming from the 

 stars must, as a ride ha\(' ti-avelhMl ihrough immense gaseous 

 envelopes and sulfered i-ay-ciii'\ ing and anomalous dispersion, just 

 as well as tlie light from the Sim. 



Taking for granted tliat most of tlie \ isible slai-s are rotat'tiKj 

 gaseous bodies, with or without a solid core, we must suppose 

 them to have a structure, describable l)y surfaces of discontinuity 

 with waves and \ortices, and rcsend)ling the {)eculiar structure of 

 the Sun, by w iiich it has pro\ed possible to ex|»lain solar phenomena'). 

 Consecpiently, the stars too give existence to "ii-regular tields of 

 radiation" rotating with them. Our line of sight contijuially cuts 

 other parts of the refracting mass; it may i)ass closely along sur- 

 faces of discontinuity, now on the one, now on the other side 

 of them; so the light reiudiing us must vary in strength and in 

 composition. 



The vai-iabiiity of many stars is very likely to result from this 

 cause: and frou) the same principle it necessarily follows that their 

 spectral lines should be liable to every kind of change in place and 

 in appearance. 



In many cases where the application of Doppler's principle leads 

 to very unsatistactory conclusions, the dispersion bands atford a 

 })lain solution. TiCt us consider, for instance, the si)ectrum of flOi-ionis. 

 In this spectrum rapi<l cliauges in the position of the lines had 

 been observed by Dkslanduks (IDOO). who concluded tVom them 

 that (\ Orionis was a spectroscopic binary having a I'cvolviug ])eriod 

 of ISVl days. Some obserxations made by J. Makt.mann ') did not 

 agree with this period. Professor Hartmann, therefore, sidnuitted the 



1) Proc. Roy. Acad. VII. ).. liO- 147 (1904). 



^) Proc. Roy. Acad. Amst. V, p. 162-171; 589-602: VI, p. 270-302 (1903). 



3) .1. Haktmann. Untersucliimgen fiber das Spectrum und die Bahn voii ^ Orionis. 



SitziuigslxT. der Kün. Preuss. Akad. d. Wissenschaften, XIV, S. 527—542, Marz 1904. 



