Double Stars. 145 



equal distances from their former position,, and travelling in 

 company on an unknown and mysterious way through the vast 

 expanse of creation. It is well-known that the term " fixed" 

 is most incorrectly applied to the stars, a great proportion of 

 which are found to change their places in the sky year after 

 year, with a slow but steady progress, the combined result, it 

 is believed, of their own movement, and a similar motion on the 

 part of our sun. How this "proper motion," as it is called, 

 can affect a pair of stars equally, and thus show their com- 

 parative nearness to each other, while yet they exhibit no ap- 

 preciable motion of revolution round each other, is wholly 

 unknown, though it may perhaps point to an extraordinary 

 deficiency in mass, as compared with magnitude or luminosity j 

 but who can wonder at our ignorance of the nature of those 

 distant suns when we know so little of the constitution of our 

 own ? It is of course possible that a progressive motion in the 

 sun may cause the apparent movement of a double star in the 

 opposite direction ; but it is obvious that such an illusory dis- 

 placement would betray its cause by the inequality of its amount 

 in the two stars, unless they were nearly at the same distance 

 from the eye, on which supposition we are brought round to 

 the same point again, by this proof that they were actually 

 adjacent, a real pair. It is then by the one or the other of the 

 these two ways — by displacement relatively to each other, in 

 such a manner as to show revolution round a common centre of 

 gravity, or by displacement relatively to the ground of the 

 heavens, and common to both individuals, that a very large 

 proportion of double stars are ascertained to be mutually 

 dependent systems. 



The study of these interesting objects has been advancing 

 with rapid steps. Only about four double stars had been 

 noticed, or at least recorded, when the elder Herschel began 

 to turn his powerful reflectors upon the heavens. He soon per- 

 ceived the importance of his research, and in 1782 published 

 the first of a series of catalogues containing in all about 500 

 stars ; but it was not till the beginning of the present century 

 that, in seeking for the parallax of the stars in order to deter- 

 mine their distance, he obtained unquestionable evidence of the 

 mutual relation of some of these pairs. His son, and other 

 observers, followed in the same track, and at length W. Struve, 

 then at Dorpat, but subsequently Director of the Imperial 

 Observatory at Poulkova, near St. Petersburg, produced a 

 catalogue of 2787 double stars, lying between the North Pole 

 and 15° of south Declinations, which has since been universally 

 regarded as the great authority for that portion of the sky. 

 According to him there were, in 1837, fifty-eight systems 

 within his limits, known to be undoubtedly binary, thirty-nine 



