DOUBLE STARS. 285 



tween the principal star and its companion is far from being 

 peculiar to the multiple stars Circumstances which we find 

 to be frequent, are not on that account necessary conditions 

 of the phenomena ; whether relating to a periodical change 

 of light, or to the revolution in partial systems round a 

 common centre of gravity. A careful examination of the 

 bright double stars (and colour can be determined even in 

 those of the 9th magnitude) teaches that, besides white, 

 all the colours of the solar spectrum are to be found in the 

 double stars, but that the principal star, whenever it is not 

 white, approximates in general to the red extreme (that of 

 the least refrangible rays), but the companion to the violet 

 extreme (the limit of the most refrangible rays). The reddish 

 stars are twice as frequent as the blue and bluish ; the white 

 are about 2^ times as numerous as the red and reddish. It 

 is moreover remarkable that a great difference of colour is 

 usually associated with a corresponding difference in bright- 

 ness. In two cases in Bootis, and y Leonis which, 

 from their great brightness can easily be measured by 

 powerful telescopes, even in the day-time, the former con- 

 sists of two white stars of the 3rd and 4th magnitudes, 

 and the latter of a principal star of the 2nd, and of a 

 companion of the 3 -5th, magnitude. This is usually called 

 the brightest double star of the northern hemisphere, whereas 

 a Centauri ** and <* Crucis, in the southern hemisphere, sur- 



24 " This superb double star (a. Cent.) is beyond all com- 

 parison the most striking object of the kind in the heavens, 

 and consists of two individuals, both of a high ruddy or orange 

 colour, though that of the smaller is of a somewhat more 

 sombre and brownish cast." (Sir John Herschel, Observa- 

 tions at the Cape of Good Hope, p. 300.) And, according 

 to the important observations taken by Captain Jacob, of the 

 Bombay Engineers, between the years 1846 and 1848, the 

 principal star is estimated of the 1st magnitude, and the 

 satellite from the 2*5th to the 3rd magnitude. (Transact, oj 

 the Royal Soc. of Edinb., vol. xvi. 1849, p. 451.) 



