204 cosmos. 



of their mutual distances. Full of imagination, yet always 

 proceeding with great caution, it was not till the year 1794, 

 while distinguishing between optically and physically double 

 stars, that he threw out his preliminary suggestions as to the 

 nature of the relation of the larger star to its smaller com- 

 panion. Nine years afterward, he first explained his views 

 df the whole system of these phenomena, in the 93d volume 

 of the Philosophical Transactions. The idea of partial 

 star-systems, in which several suns revolve round a common 

 center of gravity, was then firmly established. The stupen- 

 dous influence of attractive forces, which in our solar system 

 extends to Neptune, a distance 30 times that of the earth 

 (or 2488 millions of geographical miles), and which com- 

 pelled the great comet of 1680 to return in its orbit, at the 

 distance of 28 of Neptune's semi -diameters (853 mean dis- 

 tances of the earth, or 70,800 millions of geographical miles), 

 is also manifested in the motion of the double star 61 Cygni, 

 which, with a parallax of 0"3744, is distant from the sun 

 18,240 semi-diameters of Neptune's orbit [i. e., 550,900 

 earth's mean distances, or 45,576,000 millions of geograph- 

 ical miles). But although Sir William Herschel so clearly 

 discerned the causes and general connection of the phenome- 

 na, still, in the first few years of the nineteenth century, the 

 angles of position 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 himself alludes to the doubts regarding the accu- 

 racy of the assigned periods of revolution of a Geminorum 

 (334 years instead of 520, according to Madler),* of y Vir- 

 ginis (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 in- 

 struments, and especially in micrometrical applications, have, 

 with praiseworthy diligence, laid the proper and special foun- 



* Madler, ibid., th. i., s. 255. For Castor we have two old observa- 

 tions of Bradley, 1719 and 1759 (the former taken in conjunction with 

 Pond, the latter with Maskelyne), and two of the elder Herschel, taken 

 hi the years 1779 and 1803. For the period of revolution of y Virginia, 

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



