( 327 ) 



111 order (o explain I'cHain peculiarities in llie sj)ecü'a of Novae 

 the principle of anomalous dispersion lias already been applied Uy 

 H. Ebkkt ^). A characteristic of those sjiectra, viz. the |)resence of 

 double lines consisting of a bright and a dark component, the bright 

 one being displaced towards the red, the dark one towards the violet, 

 is very suggestively explained by this author in connection witii the 

 theory of Skkligkh. According to this theory the appearance of a 

 Nova results from a dark or taintly luminous celestial body entering 

 at a great velocity into a cosmic nebula. During this process the 

 front part of the star's surface will become excessively heated and 

 luminous, and a dense gaseous atmosphere will l)e formed, in which, 

 as Ebkrt shows, the incurvation of the rays must necessarily be such 

 as to cause the dispersion bands appearing in the spectrum to be 

 hr'Kilit on the red-facing and <lmi' on the violet-facing side of the 

 absorption lines. 



Ehkrt expresses the opinion that displacements and duplications 

 of lines in the spectra of many variables of short period might be 

 explained in a similar way, i.e. by admitting that the radiating power 

 of sncli bodies is very unequal in difTerent i)arts of their surface, 

 and that they are surrounded by dense atmospheres. Their rotation 

 will then cause us to see, as it were, the phenomena of the Novae 

 periodically rej)eated. 



In certain cases this interpretation may undoubtedly account for 

 the peculiarities observed in the spectra of \^ariables; nevertheless 

 we cannot generalize the idea without meeting with some serious 

 difficulties. First, it is not easy to form a clear conception of I lie 

 physical conditions })revailing in a star, the incandescent surface of 

 which is supposed to contain permanently large regions radiating 

 very much less than the rest. The Sun with its spots may certainly 

 not be adduced as an analogous case. Moreover, there are jdenty of 

 instances that in the spectrum of a variable, bright bands ajipear at 

 the violet side, dark bands at the red side of the absorption lines, 

 i.e. just the reverse of the phenomenon presented by the Novae; 

 and it liapi)ens that with one and the same star bright and dark 

 dispersion bands change places in course of time with respect to the 

 average position of the absorption lines. This occurs e.g. in the 

 spectrum of Mira Ceti, as will appear when comparing the obser- 

 vations made l)y Vogel and Wilsino in iSDti (Sitznngsber. derHerl. 

 Akad. XVII) with those made by Campbell in 1898 (Astroph. .lourn. 

 IX, p. 31) and by Stebbins in \\)m (Astroph. Joiiin. XVIII, |). 341); 



1) H. EBEfiT, Uelter (Tie Sprklren der neuen Slerne. Aslron. Nachr. Nr. 3017. 

 Bd. I(i4. |). i\:>, 1V)0:3. 



22 

 Proceediuss Royal Acad. Amsterdam. Voi. Vii. 



