OF THE COSMOS. PROPER MOTION OF STARS. 181 



These comparisons led, as early as the middle of the last 

 century, to the recognition of the proper motion of the fixed 

 stars generally; but we are indebted for more exact and 

 numerical determinations of this class of phenomena, first to 

 the great work of William Herschel in 1783, founded on 

 Flamsteed's observations ( 293 ), and since in a far higher de- 

 gree to Bessel's and Argelander's comparison of Bradley' s 

 Star Positions for 1755 with later catalogues. 



The discovery of the proper motion of the fixed stars, is 

 of so much the higher importance to physical astronomy, 

 since it has led to the recognition of the movement of 

 our own solar system through star-filled space, and even to 

 the exact knowledge of the direction of this movement. 

 This is a fact of which we could never have become aware, 

 if the progressive proper motion of the fixed stars had been 

 so small as altogether to escape our measurement. The 

 zealous endeavour to investigate the quantity and direction 

 of this movement, together with the parallax of the fixed 

 stars and their distance, by stimulating the improvement of 

 arc- graduation and micrometric apparatus combined with 

 optical instruments, has eminently contributed to the ad- 

 vance of observing astronomy to the point to which, (espe- 

 cially since 1830), it has been raised by the judicious 

 employment of large meridian-circles, refractors, and helio- 

 meters. 



The quantity of proper motion which has been measured 

 in different stars varies, as I have remarked in the beginning 

 of this section, from the 20th part of a second to almost 8 

 seconds. The brighter stars have in many cases a less 

 motion than stars of the 5th, 6th, and 7th magnitudes. ( 294 ) 

 The 7 stars which have shewn unusually great proper 



