PORTION OF THE COSMOS. MULTIPLE STARS. 209 



It is a well-known optical phenomenon, that a faint white 

 light appears green, when a strong (intense) red light is 

 brought near to it ; and that white light becomes blue, when 

 the surrounding stronger light is yellowish. Arago, however, 

 cautiously and justly remarked, that, although the green or 

 the blue colour of the companion may sometimes be the result 

 of contrast with the brighter star, yet that the actual existence 

 of green or of blue stars is by no means to be denied. ( 348 ) 

 He gives instances in which a bright white star (1527 

 Leonis, 1768 Can. ven.) is accompanied by a small blue 

 star; cites a double star (b Serp.), in which both the prin- 

 cipal star and its companion are blue; ( 349 ) and proposes 

 a mode of examining whether the contrasted colour is merely 

 subjective, by covering the principal star in the telescope, 

 when the distance permits, by a wire, or by a diaphragm. 

 Usually it is the smallest star only which is blue; it 

 is otherwise, however, in the double star 23 Orionis 

 (696 of Struve's catalogue, p. Ixxx), in which the principal 

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

 multiple stars, the different coloured suns are often sur- 

 rounded by planets invisible to us, such planets must be 

 variously illuminated, having their white and blue, or their 

 red and green days.( 350 ) 



As we have already seen ( 351 ) in a preceding section, that 

 the periodical variability is not necessarily associated with a 

 red or reddish colour, so also neither is colour in general, 

 nor a contrasted diversity of colour in the principal star and 

 its companion in particular, a characteristic of double stars. 

 Circumstances, which we find to be frequent, are not there- 

 fore general and necessary conditions of the phenomena, 

 whether of the periodical variation of the light of stars, or 



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