M. Arago on Double Stars, 4Bf 



where the lesser star appears so, only on account of its much 

 greater distance. Whoever is fortunate enough to discover such 

 a combination, if he can furnish himself with a perfect micro- 

 meter, of a somewhat considerable magnifying power, will dc* 

 termine the real distance of the greater star from the earth ; pro*, 

 vided, always, that this distance does not surpass thai which will 

 be traversed by the light in thirty years. It is then indeed pos- 

 sible, in spite of the accuracy of the method, that we then only 

 obtain a limit on this side of which the star will not be situated. 

 But, when it is remembered that each year contains 365J 

 days, that each day is composed of 86,400 seconds, that during 

 each of these periods light moves 80,000 of leagues, we shall per- 

 ceive how prodigious this inferior limit of distance is, when a 

 ray of light will not traverse it in less than thirty years ! ! 



6. The observations of Double Stars, properly so called, may serve one 

 day to determine either the distances of these binary groups from 

 the earth, or to fix a maximum or minimum limit beyond which 

 they cannot be placed. 



The method of parallaxes has not hitherto determined more 

 than the limit of distance (minimum distance) on this side of 

 which the observed stars are not. Thus the angular elevations 

 of 61 of the Swan, stated in page 20, have placed the two stars 

 which compose this group of 412,000,000 times, at least, more 

 distant than the sun. But that which it is necessary to add to 

 this inferior limit to know the real distance, remains wholly un- 

 known. If any one, for example, were to choose to suppose that 

 the true distance of 61 of the Swan were equal to 100,000,000 

 of times more than the inferior limit deduced by the method of 

 parallaxes, no one could contradict him, for this number is not 

 more inconipatible with the observations than a number a mil- 

 lion of times smaller, or than a number a million of times 

 greater ! In this state of the science, it was exceedingly desira- 

 ble to discover a method of placing a superior limit by the side 

 of the inferior one previously discovered. And this method 

 may, sooner or later, be deduced from the observations upon the 

 double stars, according to a process we now proceed to disclose. 



When the curve (we shall suppose it exactly circular) which 



