216 A CENTURY'S PROGRESS IN ASTRONOMY. 



believe that at one part of the sky our view 

 was limited by extinction, while at another a 

 clear view right through the Galaxy could be 

 had ; and by Robert Grant (1814-1892), director 

 of the Glasgow Observatory, who showed that, 

 were the theory true, the Galaxy should present 

 a uniform appearance throughout its course. 

 On the whole, Struve's theory was no improve- 

 ment on Herschel's ; for, as Encke pointed out, 

 Struve's theory was built on five assumptions, 

 all of which were questionable. 



At the time of Struve's investigation Madler, 

 at Dorpat, was engaged in an attempt to solve 

 the question of the construction of the heavens 

 by quite another method, that of stellar proper 

 motion. He determined to investigate the sub- 

 ject of proper motion in order to discover the 

 central body of the Milky Way. If such a 

 centre existed, however, the motions near it 

 would be somewhat different from those in the 

 Solar System. In our Solar System the planets 

 nearest the Sun move swiftest, owing to the 

 strength of the force of gravitation. In the 

 Sidereal System, on the other hand, the move- 

 ments at the centre, as Madler pointed out, 

 would be slowest. As there would be no very 

 large preponderating body, the mutual attrac- 

 tions of the different stars would cause the bodies 



