272 COSMOS. 



in mutual attraction and reciprocal action, and thus constitute 

 a particular, isolated, sidereal system. The former have long 

 been called optically, the latter physically, double stars. By 

 reason of their great distance, and ths slowness of their ellip- 

 tical 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 connexion. In sections II. and 

 III. I have already treated of the difficulty of separating 

 by the naked eye adjacent stars, with the very unequal in- 

 tensity of light, of the influence of the higher brilliancy and 

 the stars' tails, as well as of the organic defects which pro- 

 duce indistinct vision. 



Galileo, without making the double stars an especial object 

 of his telescopic observations (to which his low magni- 

 fying powers would have proved a serious obstacle), 

 mentions (in a famous passage of the Giornata terza of his 

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

 use which astronomers might make of optically double stars 

 (quando si trovasse nel telescopic qualche picciolissima stella 

 vicinissima ad alcuna delle maggiori) for determining the 

 parallax of the fixed stars . 3 As late as the middle of the 



3 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 Fixtern-Systeme, th. ii. s. 36.) 



3 Vide supra. Doctor Gregory of Edinburgh also, in 1675, 

 (consequently thirty- three years after Galileo's decease), re- 



