200 cosmos. 



pair of stars, the close proximity of which precluded their 

 separation by the naked eye (as in the case of Castor, a 

 Lyra;, (3 Ononis, and a Centauri), this designation* naturally 

 comprised two classes of multiple stars : firstly, those which, 

 from their incidental position in reference to the observer, 

 appear in close proximity, though in reality widely distant 

 and belonging to totally different strata ; and, secondly, those 

 which, from their actual proximity, are mutually dependent 

 upon each other in mutual attraction and reciprocal action, 

 and thus constitute a particular, isolated, sidereal system. 

 The former have long been called optically, the latter phys- 

 ically, double stars. By reason of their great distance, and 

 the slowness of their elliptical motion, many of the latter are 

 frequently confounded with the former. As an illustration 

 of this fact, Alcor (a star which had engaged the attention of 

 many of the Arabian astronomers, because, when the air is 

 very clear, and the organs of vision peculiarly sharp, this small 

 star is visible to the naked eye together with in the tail of 

 Ursa Major) forms, in the fullest sense of the term, one of 

 these optical combinations, without any closer physical con- 

 nection.* In sections II. and III. I have already treated of 

 the difficulty of separating by the naked eye adjacent stars, 

 with the very unequal intensity of light, of the influence of 

 the higher brilliancy and the star's tails, as well as of the 

 organic defects which produce indistinct vision. 



Galileo, without making the double stars an especial ob- 

 ject of his telescopic observations (to which his low magni- 

 fying powers would have proved a serious obstacle), men- 

 tions (in a famous passage of the Giornata terza of his Dis- 

 courses, which has already been pointed out by Arago) the 

 use which astronomers might make of optically double stars 

 (quando si trovasse nel telescopio qualche picciolissima stella 

 vicinissima ad alcuna delle maggiori) for determining the 

 parallax of the fixed stars.i As late as the middle of the 

 last century, scarcely twenty double stars were set down in 

 the stellar catalogues, if we exclude all those at a greater 



* Vide supra. As a remarkable instance of acuteness of vision, we 

 may further mention that Mostlin, Kepler's teacher, discovered with the 

 naked eye fourteen, and some of the ancients nine, of the stars in the 

 Pleiades. (Madler, Untersuch. uber die Fixslern-Systeme, th. ii., s. 36.) 



t Vide supra. Dr. Gregory, of Edinburgh, also, in 1675 (consequent- 

 ly thirty-three years after Galileo's decease), recommended the same 

 parallactic method. See Thomas Birch, Hist, of the Royal Soc, vol. 

 iii., 1757, p. 225. Bradley (1748) alludes to this method at the conclu- 

 sion of his celebrated treatise on Nutation. 



