223 COSMOS. 



the variabiles Corome, Virginis, Cancri, et Aquarii. The star 

 X Cygni likewise presents very great fluctuations at its maximum. 

 That the periods of the variable stars are very irregular 

 has been long known; but that this variability, with all its 

 apparent irregularity, is subject to certain definite laws, 

 was first established by Argelander. This he hopes to be 

 able to demonstrate in a longer and independent treatise of 

 his own. In the case of x Cygni he considers that two pertur- 

 bations in the period the one of 100, the other of 8 are 

 more probable than a single period of 108. Whether such 

 disturbances arise from changes in the process of light which 

 is going on in the atmosphere of the star itself, or from the 

 periodic times of some planet which revolves round the 

 fixed star or sun x Cygni, and by attraction influences the 

 form of its photosphere, is still a doubtful question. The 

 greatest irregularity in change of intensity has unquestionably 

 been exhibited by the variabilis Scuti (Sobieski's shield). 

 For this star diminishes, from the 5 -4th, down to the 9th 

 magnitude ; and moreover, according to Pigott, it once totally 

 disappeared at the end of the last century. At other times 

 the fluctuations in its brightness have been only from the 

 6 -5th to the 6th magnitude. The maximum of the variations 

 of x Cygni have been between the 6- 7th and 4th magnitude ; 

 of Mira, from the 4th to the 2'lst magnitude. On the other 

 hand, in the duration of its periods d Cephei shows an ex- 

 traordinary, and indeed of all variable stars the greatest 

 regularity, as is proved by the 87 minima observed between 

 the 10th of October, 1840, and 8th of January, 1848, and 

 even later. In the case of Auriga?, the variation of its 

 brilliancy discovered by that indefatigable observer, Heis, 

 of Aix-la-Chapelle, 10 extends only from the 3 -4th to the 4'5th 

 magnitude. 



w Argelander, in Schumacher's Astron. Nachr., bd. x.xvi, 

 (1848), no. 624, s. 369. 



