M . Arago on Double Stars. 38 



us to an inferior one, that is to say, to a distance on this side, of 

 which the star assuredly cannot be placed. 



Every one may now understand how brilliant those discoveries 

 may be that will reward the astronomer, who, in modifying the 

 means of observation of the double stars actually known, shall 

 assign, with a new accuracy, the durations of the ascending and 

 descending semi-revolutions of the stellary satellites. The dis- 

 covery of the distance of the stars, and the determination of 

 their masses, will become the prize of such merit ! 



7. Concerning tlie Colours observed in the Multiple Stars. 



When we remark, in the catalogue of double stars, that there 

 are so many binary combinations of red and greenish-blue, and 

 of yellow and blue, it naturally occurs, that the blue and greeu 

 tints of the lesser star are not the real shades, but the result of 

 an illusion, the simple effect of contrast. This opinion might 

 be supported by the observations which are found in all treatises 

 on optics, concerning accidental colours. In these works it is 

 stated, that a feeble white light appears green on the approach 

 of a strong red light ; and that it passes into a blue when the 

 neighbouring strong light is yellowish. These combinations 

 were, so frequently, those which exhibited themselves between 

 the more and the less brilliant parts of the double stars, that 

 we might think ourselves authorised to regard the coincidence 

 of the two phenomena that which really happened. It is, how- 

 ever, also true, that a great number of exceptions occur, and 

 they certainly ought not to be neglected. Thus it often hap- 

 pens, that a lesser blue star may be the companion of a bril- 

 liant white one. Number thirty-eight of Gemini is an ex- 

 ample of this ; « of the Lion is a second, and there are others. 

 In these instances there is no red star, and, therefore, the phe- 

 nomenon of contrast is out of the question. Consequently, the 

 blue tint cannot be considered an illusion. Blue, therefore, is 

 the real colour of some stars. This consequence flows also 

 directly from the observation of ^ of the Serpent ; for, in this 

 group, both the greater and the smaller are blue. Hence, 

 nearly ten years ago, we had many doubts whether the notion 

 of contrast satisfactorily accounted for all the facts. 



Sometimes, unquestionably, it is the real cause ; the brilliant 

 VOL. xvir. NO. XXXIII. — July 1834. c 



