286 COSMOS. 



pass all the other double stars in brilliancy. As in f 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 25 several times made men- 

 tion of the somewhat irregular variability of lustre in the 

 orange-coloured principal star in Herculis. Moreover, the 

 fluctuation in the brightness of the nearly equal yellowish 

 stars (of the 3rd magnitude) constituting the double star 

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

 probably indicates a very slow rotation of both suns upon 

 their axes. 26 Whether any actual change of colour has ever 

 taken place in double stars (as, for instance, in y Leonis and 

 7 Delphini) ; whether their white light becomes coloured, 

 and on the other hand, whether the coloured light of the 

 isolated Sirius has become white, still remain undecided 

 questions. 27 Where the disputed differences refer only to 

 faint tones of colour, we should take into consideration the 

 power of vision of the observer, and if refractors have not 

 been employed, the frequently reddening influence of the 

 metallic speculum. 



Among the multiple systems we may cite as ternaries, 

 | Libraa, Cancri, 12 Lyncis, 11 Monoc.) ; as quaternaries 

 102 and 2681 of Struve's Catalogue, a Andromeda?, e Lyroe : 

 in Orionis. the famous trapezium of the greater nebula of 

 Orion, we have a combination of six, probably a system 

 subject to peculiar physical attraction, since the five smaller 

 stars (6'3m. ; 7m.; Sin. ; ll'3m.; and 12m.) follow the proper 

 motion of the principal star 4 -7m. No change in their rela- 



25 Cosmos, vol. iii. p. 224 and note. 



26 Struve, uber Doppelst. nach Dorp. JBeob., s. 33. 



27 Ibid., s. 36. 



