; 262, 26j Condensation of Nzbulae : Double Stars 341 



ness or for other reasons his telescopes were unable to 

 resolve into stars (cf. fig. 104, facing p. 405). 



In both these respects therefore the structure of the 

 Milky Way appeared to him finally less simple than at 

 rst. 



263. One of the most notable of Herschel's discoveries 

 was a bye-product of an inquiry of an entirely different 

 character. Just as Bradley in trying to find the parallax of 

 a star discovered aberration and nutation (chapter x., 207), 

 so also the same problem in Herschel's hands led to the 

 d : scovery of double stars. He proposed to employ Galilei's 

 differential or double-star method (chapter vi., 129), i-i 

 which the minute shift of a star's position, due to the earth's 

 motion round the sun, is to be detected not by measuring 

 its angular distance from standard points on the celestial 

 sphere such as the pole or the zenith, but by observing the 

 variations in its distance from some star close to it, which 

 from its faintness or for some other reason might be 

 supposed much further off and therefore less affected by 

 the earth's motion. 



With this object in view Herschel set to work to find 

 pairs of stars close enough together to be suitable for his 

 purpose, and, with his usual eagerness to see and to record 

 all that could be seen, gathered in an extensive harvest 

 of such objects. The limit of distance between the two 

 members of a pair beyond which he did not think it worth 

 while to go was 2', an interval imperceptible to the naked 

 eye except in cases of quite abnormally acute sight. In 

 other words, the two stars even if bright enough to be 

 visible would always appear as one to the ordinary eye. 

 A first catalogue of such pairs, each forming what may 

 be called a double star, was published early in 1782 and 

 contained 269, of which 227 were new discoveries; a second 

 catalogue of 434 was presented to the Royal Society at the 

 end of 1784; and his last paper, sent to the Royal Astro- 

 nomical Society in 1821 and published in the first volume 

 of its memoirs, contained a list of 145 more. In addition to 

 the position of each double star the angular distance between 

 the two members, the direction of the line joining them, 

 and the brightness of each were noted. In some cases also 

 curious contrasts in the colour of the two components w^re 



