KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 60. NtO 8. 41 



and found that they have a continuous spectrum. He has also observed some traces 

 of lines in his spectrograms, but it has not been possible to perform any measures. 

 The form of most of the nebuke examined in that region makes it quite probable 

 that most of the objects are spiral nebulse, and in some cases the spiral structnre is 

 quite distinct. It does not seem implausible, as Wolf himself points out 244 , that the 

 whole cluster of nebulse has a spira) structure. Also in this case an asymmetrical 

 distribution of the nebuke belonging to the cluster is to be traced. 



3. Jeans' Theory. 



Jeans starts from the result found by Roche in 1855, that a rotating 

 gaseous mäss with high central condensation is flattened out more and more, 

 until it assumes the shape of a lens with a sharp periphery, from which, if 

 the speed increases further, matter will be ejected. If we consider the tidal forces, 

 excercised by distant stars at the rotation of the nebula, Jeans shows how a par- 

 ticle, thrown out from the periphery, instead of describing an elliptic orbit of small 

 excentricity, will, owing to the influence from distant stars, be gradually driven away 

 from the central mäss, in describing an orbit that satisfies the equation: 



r = aeP 9 + s cos (6 — rj), (11) 



viz. a deformed logarithmic spiral. 



Just as Laplace's ring will in course of time become instable and break up 

 into masses, which concentrate into planets, the filaments ejected from the central 

 mäss will also, according to Jeans, become instable, if the mäss per length-unit amounts 

 to a certain value. In these filaments nuclei will be condensed at tolerably regular 

 intervals and form a chain of detached masses in the spiral arms. We find that 

 such nuclei appear in the arms of most spiral nebula?. In the nebula M 81 Ritchey 

 thus has counted no less than 2400 nebulous stars. 



Jeans now considers M 101, for which van Maanen has found a rotation pe- 

 riod of 85000 years. For spiral nebulse he estimates the well-known expression 



to 0.32 and obtains a mean density of 4 10~ 17 . 



Further he approximates the density in the spiral arms to about 10 — n and 

 then obtains that the nuclei ought to occur at an interval of 0,03 parsec, if they 

 occur at all. In M 101 the nuclei are ill-defined, but Jeans estimates their mean 

 distance to 5" and thus finds a hypothetical parallax of 



0" ; 0007. 



The forms of spiral nebula? have been examined by von der Pahlen for M 33, 

 74 and 5 1 154 . He finds that they may be represented by logaritmic spirals. Slight 

 deviations occur, however, and it would perhaps be a desirable thing to examine 



K. Sv. Vet. Akad. Handl. Band 60. N:o 8. 6 



