280 COSM.OS. 



at Abo (1827-1835), by Encke and Galle, at Berlin (1836 

 and 1839), by Preuss and Otto Struve, in Pulkowa (since 

 the catalogue of 1837), by Madler, in Dorpat, and by Mitchell, 

 in Cincinnati (Ohio) with a seventeen-feet Munich refractor. 

 How many of these 6000 stars, which appear to the naked 

 eye as if close together, may stand in an immediate relation of 

 attraction to each other, forming systems of their own, and 

 revolving in closed orbits or, in other words, how many arc 

 so-called physical (revolving} double stars is an important 

 problem, and difficult of solution. More revolving compa- 

 nions are gradually but constantly being discovered. Ex- 

 treme slowness of motion, or the direction of the plane of the 

 orbit as presented to the eye, being such as to render the posi- 

 tion of the revolving star unfavourable for observation, may 

 long cause us to class physically double stars among those 

 which are only optically so ; that is, stars of which the proximity 

 is merely apparent. But a distinctly-ascertained appreciable 

 motion is not the only criterion. The perfectly uniform 

 motion in the realms of space, (i.e. a common progressive 

 movement, like that of our solar system, including the 

 earth and moon, Jupiter, Saturn, Uranus, and Neptune^ 

 with their satellites,) which in the case of a considerable 

 number of multiple stars has been proved by Argelander and 

 Bessel, bears evidence that the principal stars and their 

 companions stand in undoubted relation to each other in 

 separate partial systems. Madler has made the interesting 

 remark, that whereas previous to 1836, among 2640 double 

 stars that had been catalogued, there were only 58 in which 

 a difference of position had been observed with certainty, and 

 105 in which it might be regarded as more or less proba- 

 ble ; at present, the proportion of physically double stars to 

 optically double stars has changed so greatly in favour cf the 

 former, that among the 6000 double stars, according to a 

 table published in 1849, 650 are known in which a change of 



