PERIODIC STARS. 



modern discoveries in nebular astronomy offer other examples of 

 a like form. The axis of rotation of such a body might be sub- 

 ject to periodical change like the nutation of the earth's axis, so 

 that the flat side of the luminous disk might be presented more or 

 less towards the earth at different times, and when the edge is so 

 presented, it might be too thin to be visible. Such a succession 

 of phenomena are actually exhibited in the case of the rings of 

 Saturn, though proceeding from different causes. 



4. Mr. Dunn * has conjectured that a dense atmosphere sur- 

 rounding the stars, in different parts more or less pervious to 

 light, may explain the phenomena. This conjecture, otherwise 

 vague, indefinite, and improbable, totally fails to explain the 

 periodicity of the phenomena. 



5. It has been suggested that the periodical obscuration or total 

 disappearance of the star, may arise from transits of the star by 

 its attendant planets. The transits of Yenus and Mercury are 

 the basis of this conjecture. 



The transits of none of the planets of the solar system, seen 

 from the stars, could render the sun a periodic star. The magni- 

 tudes, even of the largest of them, are altogether insufficient for 

 such an effect. To this objection it has been answered that planets 

 of vastly greater comparative magnitude may revolve round other 

 suns. But if the magnitude of a planet were sufficient to produce 

 by its transit these considerable obscurations, it must be very 

 little inferior to the magnitude of the sun itself, or at all events, 

 it must bear a very considerable proportion to the magnitude of 

 the sun; in which case it may be objected that the predominance 

 of attraction necessary to maintain the sun in the centre of its 

 system could not be secured. To this objection it is answered, 

 that although the planet may have a great comparative magnitude, 

 it may have a very small comparative density, and the gravitating 

 attraction depending on the actual mass of matter, the predomi- 

 nance of the solar mass may be rendered consistent with the 

 great relative magnitude of the planet by supposing the density of 

 the one vastly greater than that of the other. The density of the 

 sun is much greater than the density of Saturn. 



6. It has been suggested that there may be systems in which 

 the central body is a planet attended by a lesser sun revolving 

 round it as the moon revolves round the earth, and in that case 

 the periodical obscuration of the sun may be produced by its pas- 

 sage once in each revolution behind the central planet. 



Such are the various conjectures which have been proposed to 

 explain the periodic stars ; and as they are merely conjectures,. 



* Phil. Trans, vol. lii. 



201 



