400 A Short History of Astronomy [Cn. xiil. 



attracted one another according to the law of gravitation, 

 so that the motion of revolution was to some extent 

 analogous to that of a planet round the sun ; if this were 

 the case, then each star of a pair should describe an ellipse 

 (or conceivably some other conic) round the other, or each 

 round the common centre of gravity, in accordance with 

 Kepler's laws, and the apparent path as seen on the sky 

 should be of this nature but in general foreshortened by 

 being projected on to the celestial sphere. The first attempt 

 to shew that this was actually the case was made by Felix 

 Savary (1797-1841) in 1827, the star being $ Ursae, which 

 was found to be revolving in a period of about 60 years. 



Many thousand double stars have been discovered by 

 the Herschels, Struve, and a number of other observers, 

 including several living astronomers, among whom Pro- 

 fessor S. W. Burnham of Chicago, who has discovered 

 some 1300, holds a leading place. Among these stars there 

 are about 300 which we have fair reason to regard as 

 binary, but not more than 40 or 50 of the orbits can be 

 regarded as at all satisfactorily known. One of the most 

 satisfactory is that of Savary's star Ursae, which is shewn 

 in fig. 1 01. Apart from the binaries discovered by the 

 spectroscopic method ( 314), which form to some extent 

 a distinct class, the periods of revolution which have been 

 computed range between about ten years and several 

 centuries, the longer periods being for the most part 

 decidedly uncertain. 



310. William Herschel's telescopes represented for some 

 time the utmost that could be done in the construction of 

 reflectors ; the first advance was made by Lord Rosse 

 (1800-1867), who after a number of less successful ex- 

 periments finally constructed (1845), at Parsonstown in 

 Ireland, a reflecting telescope nearly 60 feet in length, with 

 a mirror which was six feet across, and had consequently a 

 " light -grasp " more than double that of Herschel's greatest 

 telescope. Lord Rosse used the new instrument in the first 

 instance to re-examine a number of known nebulae, and in 

 the course of the next few years discovered a variety of new 

 features, notably the spiral form of certain nebulae (fig. 102), 

 and the resolution into apparent star clusters of a number 

 of nebulae which Herschel had been unable to resolve 



