34 M. Arago on Double Stars. 



star being red or yellow, the smaller one appears of a green or 

 blue tint. A very simple experiment is sufficient to distinguish 

 these cases from the others ; we have only to conceal the prin- 

 cipal star from our view by a thread or a small patch placed 

 on the glass. If, during the occultation of the greater star, 

 the smaller, which then appears alone, ceases to be coloured ; 

 if it becomes white, the green or bluish tint, with which it seemed 

 overspread when the two were seen simultaneously, was only an 

 illusion. When, on the other hand, the contrary happens, we 

 cannot refuse to consider these tints as real. Thus, then, the oc- 

 cultation of the greater star produces a disappearance of colour 

 in the smaller, only in a certain number of cases ; and most com- 

 monly, this occultation leaves the tint of the smaller star un- 

 altered, or, at least, induces only modifications that are insensible 

 to us. 



The existence of so may blue or green stars in the binary 

 groups, known under the name of double sta7'S, is a fact the 

 more worthy of attention, because betwixt the 60,000,000 or 

 80,000,000 of isolated stars, whose positions the astronomical 

 catalogues make known to us, we believe there is not one to 

 which there is attributed any other characters, in regard to 

 tints, than white, red, and yellow. The inherent physical con- 

 ditions, then, respecting the emission of a blue or green light, 

 seem to be met with only in the multiple stars. 



This phenomenon has been observed for too short a time * 



• We have been anxious to find out what observer it was that first re- 

 cognised the existence of blue stars. Tlie ancients only speak of white and red 

 stars. In this last class they placed Arcturus, Aldebaran, Pollux, Antarus, and 

 a of Orion, and all these remain red to the present day. To this list, and this is 

 a circumstance which is worthy of remark, they add Sirius — whose whiteness 

 is now remarked by every body. It would seem then, that with time certain 

 stars change their colour. We shall now quote the first passage known to us 

 where mention is made of blue stars. It will be found in Le Traite des Cou- 

 leurs de Mariotte, which was published in 1686. 



" II y a des etoiles," &c. " There are some stars that are very red, as the 

 eye of the Bull, and the heart of the Scorpion ; there are also others that are 

 yellow and blite :'* and again — " The stars that appear red and yellow must 

 needs be very luminous, but having their vivacity obscured by exhalations 

 which are spread over them ; and those which appear blue^ have a feeble light, 

 but pure and without exhalations." 



In the catalogue which Mr Dunlop published in 1828, there will be 

 found in the Southern Hemisphere, notice of a group that has three and a 



