184 cosmos. 



euce of observation through the improvement of instruments 

 and methods. The discovery of this motion was first ren- 

 dered practicable when the telescope was combined with 

 graduated instrmnents ; when, from the accuracy of within 

 a minute of an arc (which after much pains Tycho Brahe 

 first succeeded in giving to his observations on the island of 

 Hven), astronomers gradually advanced to the accuracy of 

 a second and the parts of a second ; and when it became 

 possible to compare with one another results separated by a 

 long series of years. Such a comparison was made by Hal- 

 ley with respect to the positions of Sirius, Arcturus, and Al- 

 debaran, as determined by Ptolemy in his Hipparchian cat- 

 alogue, 1844 years before. By this comparison he consid- 

 ered himself justified (1717) in announcing the fact of a 

 proper motion in the three above-named fixed stars.* The 

 high and well-merited attention which, long subsequent even 

 to the observations of Flamstead and Bradley, was paid to 

 the table of right ascensions contained in the Triduum of 

 Romer, stimulated Tobias Mayer (1756), Maskelyne (1770), 

 and Piazzi (1800) to compare Romer's observations with 

 more recent ones.f The proper motion of the stars was in 

 some degree recognized as a general fact, even in the mid- 

 dle of the last century ; but for the more precise and numer- 

 ical determination of this class of phenomena we are in- 

 debted to the great work of William Herschel in 1783, found- 

 ed on the observations of Flamstead,! and still more to Bes- 

 sel and Argelander's successful comparison of Bradley's " Po- 

 sitions of the Stars for 1755" with recent catalogues. 



The discovery of the proper motion of the fixed stars has 

 proved of so much the greater importance to physical astron- 

 omy, as it has led to a knowledge of the motion of our own 

 solar system through the star-filled realms of space, and, in- 

 deed, to an accurate knowledge of the direction of this mo- 

 tion. We should never have become acquainted with this 

 fact if the proper progressive motion of the fixed stars were 

 so small as to elude all our measurements. The zealous at- 

 tempts to investigate this motion, both in its quantity and 

 its direction, to determine the parallax of the fixed stars, and 



* Halley, in the Philos. Transact, for 1717-1719, vol." xxx., p. 736. 

 The essay, however, referred solely to variations in latitude. Jacques 

 Cassini was the first to add variations in longitude. (Arago, in the An- 

 nuaire pour 1842, p. 387.) 



t Delambre, Hist, de V Astron. Moderne, t. ii., p. 658. Also in Hist 

 de I' Astron. au V&lme Sidcle, p. 448. 



t Philos. Transact., vol. lxxiii., p. 138. 



