M. Arago on Double Stars. "SQ 



36th of Andromeda. The distance between the two centres was 0".7 in 1831. 



^ of the Crown. . . • 0". 8 in 1830. 



^ of the Crown. 1 ".8 in 1830. 



y of the Crown. One of the most difficult to distinguish, as much on account 

 of the extreme approximation of the two stars, as on ac- 

 count of their great difference of intensity. 



I of the Ram. Very difficult to separate. 



» of Hercules. Do. do. do. 



T of Serpentarius. The telescope of Dorpat itself does not now distinguish 

 them. From older observations, however, we have 

 learnt that this is a double star. 



That these observations might be complete, it was important 

 not to forget to point out the great advantage which may now 

 be derived, by the examination of double stars, in the trying of 

 great telescopes. On all accounts, we are persuaded that the 

 importance of the application will be recognised, when it is con- 

 sidered that the kind of these instruments which is indispensa- 

 ble for all the great observatories, costs eight hundred, a thou- 

 sand, or even sixteen hundred pounds, and this independent of 

 the expense of mounting. 



9. Of the part which the doctrine of Probabilities ha^ fulfilled in the 

 question of dovhle stars. 



The calculations of probabilities has enriched astronomy with 

 a great number of very remarkable results. Hitherto, however, 

 the doctrine has not assumed, either in teaching or in elementary 

 works, the place that is due to it. It would appear that there 

 has been a dread of darkening the truths of the science, whose 

 demonstration rests on the immediate union of direct observa- 

 tions, by associating them with deductions, which, without pos- 

 sessing altogether the same certainty, nevertheless merit, in the 

 mean time, to be held in high estimation. We may add, that 

 we do not know a question more fit than that of double stars, to 

 demonstrate how far observers would err in despising the lessons 

 taught by the calculation of probabilities. 



As far back as 1767, a distinguished astronomer, John Mit- 

 chel, struck with the irregular distribution of the stars in the 

 firmament, examined if it were possible to believe that this dis- 

 tribution was the effect of accident. He took, as an example, 

 the constellation of the Pleiades. 



This group is composed of six principal stars. On the whole 



