340 HUMBOLDT S COSMOS : 



We can but advert, in passing, to the curious observations 

 of Struve, Arago, and Herschel, on the contrasted and com- 

 plementary colours of many of the double stars, and must 

 follow our author hastily through the further questions of 

 the distance of the fixed stars ; of the proper motion of our 

 own Sun and other stars in space ; and of the existence of 

 a common centre of gravity and revolution for the whole 

 sidereal system, to which our Solar system belongs. To some 

 of these topics we have already had occasion to allude in the 

 way of illustration. The determination of the distance of 

 certain of the fixed stars is one of the achievements of late 

 years. It fulfills a desire of much longer date, which had 

 been rendered unavailing by imperfection of instruments, the 

 difficulty of separating the parallactic and proper motions of 

 stars, and other causes. Abstractedly, the problem of find- 

 ing the parallax is one of simple trigonometry, and astrono- 

 mers had already provided the measure of the diameter of 

 the earth's orbit as a base for the operation. Yet even this 

 vast base, of nearly 200' millions of miles, failed to render 

 any assured angle of parallax to the earlier instruments em- 

 ployed in the attempt. And it was not until the workshop of 

 Munich had furnished its admirable refractors, and microme- 

 ters been added to them capable of designating the 60,,000th 

 part of an inch, that the great result was unequivocally ob- 

 tained. After three years of patient observations, begun in 

 1837, Bessel announced the discovery of the parallax of 61 

 Cygni ; and the wonderful conclusion as to distance founded 

 upon it, to which we have before referred. The certainty of 

 the fact was fully attested by the exact correspondence of the 

 annual changes in the place of the star ; the parallactic varia- 



addition has been made to our catalogue of double stars, in the discovery of a 

 companion to the splendid star of Sirius. This had before been conjectured 

 by Bessel, from his notice of a periodical variation in the right ascension of 

 Sirius; but the discovery was finally made by Mr. Clarke of the Harvard 

 Observatory, Massachusets. 



