HISTORY OF ASTRONOMY. 



torial belt could not be situate close to the surface of tlie 

 planet. In 1723, Halley remarked that the dusky line 

 which in 1720 he observed to accompany the inner edge 

 of the ring across the disc, continued close to the same, 

 though the breadth of the ellipse had considerably in- 

 creased since that time. M. Struve remarks that the 

 projection of the obscure ring upon the body of the planet 

 agrees with the position assigned by the earlier observers 

 to the equatorial belt, and that no trace of the latter is 

 visible upon the planet in the present day. He therefore 

 arrives at the conclusion that the obscure ring has ex- 

 isted around the planet f at least since the time when 

 telescopes were first constructed of sufficient optical 

 power to exhibit the belts upon his disc. 



The breadth of the space which separates the inner edge 

 of the interior bright ring and the body of the planet hav- 

 ing exhibited a considerable discordance as measured by 

 himself in 1851, and by his father in 1826, M. Otto Struve 

 instituted a careful examination of the various measure- 

 ments of preceding astronomers relative to the dimensions 

 of the planet and the rings. By a comparison of the micro- 

 metrical measures of Huygens, Cassini, Bradley, Her- 

 schel, "W. Struve, Encke and Galle, with the correspond- 

 ing measures executed by himself, he found that the 

 inner edge of the interior bright ring is gradually ap- 

 proaching the body of the planet, while at the same time 

 the total breadth of the two bright rings is constantly 

 increasing. The earlier measures employed in this com- 

 parison are, however, founded upon certain assumptions 



