278 COSMOS. 



But although Sir William Herschel so clearly discerned the 

 causes and general connexion of the phenomena, still, in the 

 first few years of the nineteenth century, the angles of posi- 

 tion derived from his own observations, owing to a want of 

 due care in the use of the earlier catalogues, were confined 

 to epochs too near together to admit of perfect certainty in 

 determining the several numerical relations of the periodic 

 times, or the elements of their orbits. Sir John Herschel him- 

 self alludes to the doubts regarding the .accuracy of the assigned 

 periods of revolution of at Geminorum (334 years instead of 

 520, according to Madler), 10 of y Virginis (708 instead of 169), 

 and of y Leonis (1424 of Struve's great catalogue), a splendid 

 golden and reddish-green double star (1200 years). 



After William Herschel, the elder Struve (from 1813 to 

 1842), and Sir John Herschel (from 1819 to 1838), availing 

 themselves of the great improvements in astronomical instru- 

 ments, and especially in micrometrical applications, have, 

 with praiseworthy diligence, laid the proper and special 

 foundation of this important branch of astronomy. In 1820, 

 Struve published his first Dorpat Table of double stars, 

 796 in number. This was followed in 1824 by a second, 

 containing 3112 double stars, down to the 9th magnitude, 

 in distances under 32", of which only about one-sixth had 

 been before observed. To accomplish this work, nearly 

 120000 fixed stars were examined by means of the great 

 Fraunhofer refractor. Struve's third Table of multiple stars 

 appeared in the year 1837, and forms the important work 

 Ktellarum compositarum Mensurce micrometricce. u It contains 



10 Madler, ibid., th. i. s. 255. For Castor we have two 

 old observations of Bradley, 1719 and 1759 (the former taken 

 in conjunction with Pond, the latter with Maskelyne), and 

 two of the elder Herschel, taken in the years 1779 and 1803. 

 For the period of revolution of y Virginis, see Madler, 

 Fixstern-Syst.. th. ii. s. 234-40, 1848. 



11 Struve, Mensura microm., pp. 40 and 234-248. On the 



