VARIABLE STARS. 229 



has been established by Argelander, from which all the 

 maxima can be so deduced that the probable error in a long 

 period of variability, extending to 33 Id. 8h. does not in the 

 mean exceed 7 days, while, on the hypothesis of an uniform 

 period, it would be 15 days. 



The double maximum and minimum of /3 Lyra?, in each 

 of its periods of nearly 13 days, was from the first correctly 

 ascertained by its discoverer, Goodricke (1784); but it has 

 been placed still more beyond doubt 13 by very recent obser- 

 vations. It is remarkable that this star attains to the same 

 brightness in both its maxima ; but in its principal minimum 

 it is about half a magnitude fainter than in the other. Since 

 the discovery of the variability of /3 Lyra?, the period in a 

 period has probably been on the increase. At first the vari- 

 ability was more rapid, then it became gradually slower ; and 

 this decrease in the length of time reached its limit between 



1751 Sep. 9-76 + 331 d -3363 E. 



+ 10 d -5, sin. ( 3 T 6 r o0 E + 86 23') + 18 d -2,sin. (ffE + 23142') 

 + 33 d -9, sin. (f| E + 170 19') + 65 d -3, sin. (ff E + 6 37') 

 where E represents the number of maxima which have oc- 

 curred since Sept. 9, 1751, and the co- efficients are given 

 in days. Therefore, for the current year (E being = 109), 

 the following is the maximum : 



1751 Sep. 9-76 + 36115 d '65 + 8 d> 44 12 d '24. 

 + 18 d -59 + 27 d '34 = 1850 Sep. 8 d -54. 



" The strongest evidence in favour of this formula is, that it 

 represents even the maximum of 1596, ( Cosmos, vol. ii. p. 713,) 

 which, on the supposition of a uniform period, would deviate 

 more than 100 days. However, the laws of the variation of 

 the light of this star appear so complicated, that in par- 

 ticular cases c. g. for the accurately observed maximum 

 of 1840 the formula was wrong: bv manv davs (nearly 

 twenty-five)." 



13 Compare Argelander's essay written on the occasion of 

 the centenary jubilee of the Konigsberg University, and en- 

 titled, De Stella ft Lyra Variabili, 1844. 



