PORTION OF THE COSMOS. MULTIPLE STARS. 207 



double stars to be included by us among optically double 

 stars in which the proximity is only apparent. But a dis- 

 tinctly recognised measurable motion, such as we have been 

 speaking of, is not the only criterion; Argelander and 

 Bessel have shown, in a considerable number of multiple 

 stars, a perfectly equal proper motion in space (i. e. a com- 

 mon progressive movement, such as that of our own solar 

 system, including, together with the Sun, all its planets and 

 satellites), which testifies in favour of the principal stars and 

 their companions being respectively connected with each 

 other by a true and actual relation, forming separate partial 

 systems. Madler has made the interesting remark that, 

 whereas in 1836, among 2640 catalogued double stars, 

 there were only 58 in which a difference of relative position 

 had been observed with certainty, and 105 in which such 

 a difference could be regarded as indicated with a greater or 

 less degree of probability, the proportion of physical to 

 optical double stars is now so changed in favour of the 

 first, that among 6000 multiple stars there are, according 

 to a Table published in 1849, six hundred and fifty ( 343 ) 

 in which an alteration of relative position can be demon- 

 strated. The earlier ratio gave 1 in 16, the latter one 

 already gives 1 in 9, for the proportion of cases in which 

 the motions of the principal star and its companion show 

 these celestial bodies to be physically double. 



The relative distribution of binary star-systems, not only 

 in the celestial spaces generally, but even simply on the ap- 

 parent heavenly vault, has as yet been but little examined 

 numerically. In the Northern Hemisphere, double stars are 

 most frequent in the direction of certain constellations (An- 

 dromeda, Bootes, the Great Bear, the Lynx, and Orion), 



