DOUBLE STARS. 277 



1783, and 1804, he has not only set down and determined 

 the position and distance of 846 double stars, 9 for the most 

 part first discovered by himself, but, what is far more impor- 

 tant than any augmentation of number, he applied his 

 sagacity and power of observation to all those points which 

 have any bearing on their orbits, their conjectured periodic 

 times, their brightness, contrasts of colours, and classification 

 according to the amount 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 companion. Nine years after- 

 wards, he first explained his views of the whole system of 

 these phenomena, in the 93rd volume of the Philosophical 

 Transactions. The idea of partial star-systems, in which 

 several suns revolve round a common centre of gravity, was 

 then firmly established. The stupendous influence of attrac- 

 tive 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 compelled the great comet 

 of 1680 to return in its orbit, at the distance of 28 of 

 Neptune's semi-diameters (853 mean distances of the earth, 

 or 70800 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 18240 semi- 

 diameters of Neptune's orbit (t. e. 550900 earth's mean 

 distances, or 45576000 millions of geographical miles). 



* Philos. Transact. for the year 1782, pp. 40-126; for 1783. 

 pp. 112-124; for 1804, p. 87. Regarding the observations 

 on which Sir William Herschel founded his views respecting 

 the 846 double stars, see Madler, in Schumacher's Jahrbuch 

 fur 1839, s. 59, and his Untersuchungen iiber die Fixstern- 

 Systeme,ih.i. 1847, s. 7. 



