206 SPECIAL RESULTS IN THE URANOLOGICAL 



reflector; they are reduced to 1830, and are arranged in 

 6 Catalogues, which contain 3346 double stars, and were 

 presented by Sir John Herschcl to the Royal Astronomical 

 Society of London, for the 6th and 9th parts of their 

 valuable memoirs. ( 341 ) In the European portion of these 

 catalogues, there are included 380 double stars which were 

 observed in 1825 by the above-named celebrated astro- 

 nomer, conjointly with Sir James South. 



We see by this historical account, how, in the course 

 of half a century, science has gradually arrived at an 

 extensive and accurate knowledge of partial, and more 

 particularly of binary, star systems existing in space. The 

 number of double stars (including those both optically and 

 physically double) may now be estimated with some degree 

 of security at 6000, including those observed by Bessel 

 with the fine Praunhofer heliometer, by Argelander ( 342 ) 

 at Abo (18271835), by Encke and Galle at Berlin 

 (1836 and 1839), by Preuss and Otto Struve at Pulkova 

 (since the Catalogue of 1837), by Madler at Dorpat, and 

 by Mitchell at Cincinnati in Ohio with a 17 feet Munich 

 Refractor. In how many of these cases the stars seen in 

 close proximity in the telescope are really connected with 

 each other by immediate relations of attraction, forming par- 

 ticular systems and revolving in closed orbits, i. e. how 

 many are what are called physically double stars, is an 

 important question, but one difficult to answer. More and 

 more revolving companions are gradually being discovered. 

 Extraordinary slowness of motion, or the circumstance of 

 the direction of the plane of the orbit, as it presents itself 

 to our eyes, being such that the position of the moving star 

 is unfavourable for observation, may long cause physically 



