284 COSMOS. 



instances in which a brilliant white star (1527 Leonis, 1768 

 Can. ven.) is accompanied by a small blue star; others, where 

 in a double star (d Serp.) both the principal and its companion 

 are blue. 81 In order to determine whether the contrast of 

 colours is merely subjective, he proposes (when the distance 

 allows) to cover the principal star in the telescope by a thread 

 or diaphragm. Commonly it is only the smaller star that is 

 blue: this, however, is not the case in the double star 23 

 Orionis (696 in Struve's Catalogue, p. Ixxx.) ; where the prin- 

 cipal star is bluish, and the companion pure white. If in the 

 multiple stars the differently coloured suns are frequently 

 surrounded by planets invisible to us, the latter, being dif- 

 ferently illuminated, must have their white, blue, red, and 

 green days. 2 * 



As the periodical variability** of the stars is, as we have 

 already pointed out, by no means necessarily connected 

 with their red or reddish colour, so also colouring in gene- 

 ral, or a contrasting difference of the tones of colour be- 



bustion at work within them; whether the colour and the excess 

 of the most refrangible rays often presented by the smaller 

 of two stars be not owing to the absorbing force of an atmo- 

 sphere developed by the action of the accompanying star, 

 which is generally much the more brilliant of the two." (Arago 

 in the Annuaire pour 1834, pp. 295-301.) 



21 Struve, Ueber Doppelsterne nach Dorpater Beobachtungen, 

 1837, s. 33-36, and Mensura microm. p. Ixxxiii., enumerates 

 sixty-three double stars, in which both the principal and 

 companion are blue or bluish, and in which therefore the 

 colours cannot be the effect of contrast. When we are forcr.d 

 to compare together the colours of double stars, as reported 

 by several astronomers, it is particularly striking to observe 

 how frequently the companion of a red or orange-coloured 

 star is reported by some observers as blue, and by others 

 as green. 



n Arago, Annuaire pour 1834, p. 302. 



* Vide supra, pp. 175-183. 



