VARIABLE STABS. 231 



twice as great a deviation. In the case of 30 Hydra?, which 

 has a period of 495 days, it is still greater, probably one-fifth. 

 It is only during the last few years (since 1840, and still 

 later) that the variable stars with very short periods have 

 been observed steadily, and with sufficient accuracy ; so that 

 the problem in question, when applied to them, is still more 

 difficult of solution. From the observations, however, which 

 have as yet been taken, less considerable deviations seem to 

 occur. In the case of v AquilaB (with a period of 7d. 4h.) 

 they' only amount to one-sixteenth or one-seventeenth of the 

 whole period; in that of & Lyrae (period 12d. 21h.) to one 

 twenty-seventh or one-thirtieth; but the inquiry is still 

 exposed to much uncertainty as regards the comparison of 

 long and short periods. Of j8 Lyras between 1700 and 1800 

 periods have been observed; of Mira Ceti, 279; of % 

 Cygni, only 145. 



The question that has been mooted, w r hether stars which 

 have long appeared to be variable in regular periods, ever 

 cease to be so, must apparently be answered in the negative. 

 As among the constantly variable stars there are some which 

 at one time exhibit a very great, and at another a very small 

 degree of variability, (as, for instance, variabilis Scuti,) so, 

 it seems, there are also others whose variability is at certain 

 times so very slight, that, with our limited means, we are 

 unable to detect it. To such belongs variabilis Corona? bor. 

 (Xo. 5236 in the Catalogue of the British Association), 

 recognized as variable by Pigott, who observed it for a 

 considerable time. In the winter of 1795-6 this star became 

 totally invisible; subsequently it again appeared, and the 

 variations of its light were observed by Koch. In 1817, 

 Harding and Westphal found that its brightness was nearly 

 constant, while in 1824 Olbers was again enabled to perceive 

 a variation in its luminosity. Its constancy now again 

 returned, and from August, 1843, to September, 1845, was 



