210 COSMOS. 



a corresponding difference in brightness. In two cases in 

 Bootis and y Leonis which, from their great brightness, 

 can easily be measured by powerful telescopes, even in the 

 daytime, the former consists of two white stars of the third 

 and fourth magnitudes, and the latter of a principal star of 

 the second, and of a companion of the 3 -5th magnitude. 

 This is usually called the brightest double star of the north- 

 ern hemisphere, whereas a Centauri* and a Crucis, in the 

 southern hemisphere, surpass all the other double stars in 

 brilliancy. As in Bootis, so also in a Centauri and y Leonis, 

 we observe the rare combination of two great stars with only 

 a slightly different intensity of light. 



No unanimity of opinion yet prevails respecting the vari- 

 able brightness in multiple stars, and especially in that of 

 companions. We have already t several times made mention 

 of the somewhat irregular variability of luster in the orange- 

 colored principal star in a Herculis. Moreover, the fluctua- 

 tion in the brightness of the nearly equal yellowish stars (of 

 the third magnitude) constituting the double star y Virginis 

 and Anon. 2718, observed by Struve (1831-1833), probably 

 indicates a very slow rotation of both suns upon their axes.J 

 Whether any actual change of color has ever taken place 

 in double stars (as, for instance, in y Leonis and y Delphini) ; 

 whether their white light becomes colored, and, on the other 

 hand, whether the colored light of the isolated Sirius has be- 

 come white, still remain undecided questions. Where the 

 disputed differences refer only to faint tones of color, we should 

 take into consideration the power of vision of the observer, 

 and, if refractors have not been employed, the frequently red- 

 dening influence of the metallic speculum. 



Among the multiple systems we may cite as ternaries, 

 Librae, Cancri, 12 Lyncis, 11 Monoc. ; as quaternaries, 

 102 and 2681 of Struve's Catalogue, a Andromedae, e Lyrae : 

 in 6 Orionis, the famous trapezium of the greater nebula of 



* " This superb double star (a Cent.) is beyond all comparison the 

 most striking object of the kind in the heavens, and consists of two in- 

 dividuals, both of a high ruddy or orange color, though that of the 

 smaller is of a somewhat more somber and brownish cast." (Sir John 

 Herschel, Observations at the Cape of Good Hope, p. 300.) And, ac- 

 cording 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 first magnitude, and the satellite from the 2'5th to 

 the third magnitude. ( Transact, of tht Royal Soc. of Edinb., vol. X'vn 

 1849, p. 451.) 



t Videtupra, p. 165, 166, and note. 



t Struve, Ueber Doppeltt. nach Dorp Beob., s. 33. $ Ibid., s. 36 



