PliOPEE MOTIONS OF STAES. 135 



propagation and aberration of light, or from the parallax 

 produced by the opposite positions of the Earth in 

 its orbit round the Sun, we find a residual annual 

 motion of the fixed stars, which includes both the trans- 

 lation of the whole solar system in space, and the actual 

 proper motions of the stars themselves. The very 

 difficult numerical separation of these two elements has 

 been rendered possible, by a careful specification of the 

 directions in which the movements of the different stars 

 take place, and by the consideration that, if they were all 

 absolutely at rest, they would appear perspectively.to recede 

 from the point towards which the Sun's course is di- 

 rected. The result of the investigation, confirmed by the 

 theory of probabilities, is, that both our solar system and 

 the stars are changing their place in space. It appears 

 from the admirable investigations of Argelander ( 102 ), (who 

 has been engaged at Abo in extending, and in carrying to 

 much greater perfection, the work commenced by William 

 Herschel and Prevost,) that the Sun is moving towards the 

 constellation of Hercules, and very probably towards a 

 point which, from a combination of the observations of 

 537 stars, was situated (equinox of 1792'5) in 257 49''7 

 Eight Ascension, and in 28 49'7 North Declination. In 

 this delicate investigation there is still great difficulty in 

 separating the absolute from the relative motion, and in 

 determining what portion belongs to the solar system only. 

 If we consider the proper motions of stars, as contra- 

 distinguished from their apparent or perspective motions, 

 their directions are various and even opposite in different 

 groups ; it is not, therefore, a necessary conclusion, either 

 lat all parts of our astral system, or that all the systems 

 rhich fill universal space, revolve around one great undis- 



