OF THE COSMOS. PERIODICALLY VARIABLE STARS. 169 



to express the perturbation of the period by a formula of 

 sines. 



8. a Cephei, E. A. 335 54', Decl. + 57 39'. Is of all 

 known stars the one which shews the greatest regularity 

 in all respects. The period of 5d. 8h. 47m. 39'5s. re- 

 presents all observations from 1784 to the present time, 

 to within the limits of errors of observation, which errors 

 may also suffice for the explanation of the small differences 

 which shew themselves in the march of the variations of 

 light. At its minimum the star is -f-th of a gradation 

 brighter than e Cephei, and at its maximum it is equal to e 

 of the same constellation ; it takes Id. 15h. to rise from 

 the minimum to the maximum, and more than double that 

 time, i.e. 3d. 18h. to return to the minimum ; after which, 

 however, it scarcely alters at all for eight hours, and only 

 quite inconsiderably for an entire day. 



9. a Herculis, K A. 256 57', Decl. + 14 84'. A very 

 red double star, whose variation of light is irregular in 

 every respect. Often it scarcely alters at all for months, 

 at other times it is five gradations brighter at its maximum 

 than at its minimum, and hence the period is also still 

 very uncertain. The discoverer assumed it at 63 days ; 

 I began by taking it at 95, until a careful calculation of 

 all my observations during seven years gave me the period 

 assigned in the text. Heis thinks that he can represent 

 the observations by a period of 184d. 9h., having two 

 maxima and two minima. 



10. CoromeK, E. A. 235 36', Decl. + 28 37'. This 

 star is only occasionally variable ; the assigned period was 

 calculated by Koch from his own observations, which are 

 unfortunately lost. 



