204 SPECIAL RESULTS IN THE URA.NOLOGICAL 



of colours, and classification according to the degree of 

 distance apart, of the double stars. Imaginative, and yet 

 always advancing with caution, he expressed himself, in the 

 year 1794, when distinguishing between optically and 

 physically double stars, in a brief and preliminary manner 

 respecting the nature of the relation subsisting between the 

 larger star and its smaller companion. Nine years after- 

 wards, he first developed the entire connection and mutual 

 dependence of the phenomena, in the 93rd volume of the 

 Philosophical Transactions. The idea of partial star-systems, 

 in which two or more suns revolve around a common centre 

 of gravity, was now firmly established. The powerful 

 dominion of attracting forces, which, in our solar system, 

 extends to Neptune, at a mean solar distance 30 times 

 greater than that of the Earth (or 2488 millions of 

 geographical miles), and even constrained the great comet 

 of 1680 to return when at a distance equal to 28 distances 

 of Neptune, or 70,800 millions of geographical miles, also, 

 reveals itself in the motion of the double star 61 Cygni, 

 which, in correspondence with a parallax of /x> 3744, is 

 18,240 distances of Neptune, or 550,900 semi-diameters 

 of the Earth's orbit, or 11,394,000 German or 45,576,000 

 English millions of geographical miles from our sun. If, 

 however, the causes and the general connection of the 

 phenomena were very distinctly recognised by "William 

 Herschel, yet in the first ten years of the 19th century, the 

 angles of position derived from his own observations, and 

 from older star-catalogues employed without sufficient care, 

 belonged to epochs too close together, to admit of the periods 

 of revolution or the elements of the orbits being derived 

 with due certainty from the several numerical values. Sir 



