200 SPECIAL RESULTS IN THE TJRANOLOGICAL 



might really be situated at very different distances, and might 

 belong to altogether different sidereal strata, and those 

 which, being actually and truly near to each other, and 

 being connected by mutual dependence or reciprocal attrac- 

 tion, form a particular system of their own. It is now the 

 custom to call the first class optically, and the second 

 physically, double stars. Yery great distance and slowness 

 of elliptic motion, may possibly cause several of. the latter 

 class to be confounded with the former. To cite here a 

 well-known object, the small star Alcor, (which received 

 much attention from Arabian astronomers, because visible 

 to the naked eye in very clear atmosphere and to persons 

 whose visual organs are very perfect,) forms, in the widest 

 sense of the term, such an optical combination as has been 

 spoken of, with % in the tail of the Great Bear. I have 

 already noticed in Sections II. and III. the difficulty of 

 separating with the unassisted eye adjacent stars of very 

 unequal intensity of light, the influence generally of such 

 inequality of light, the rays which appear to issue from 

 stars, and the organic defects which produce indistinctness 

 of vision. ( 33 ) 



Galileo, without making double stars a particular subject 

 of his telescope observations, (for which, indeed, the mag- 

 nifying powers employed by him would have been quite 

 inadequate), yet in a celebrated passage of the Giornata 

 terza of his Discourses, pointed out by Arago, mentions the 

 use which astronomers might make of optically double stars 

 (quando si trovasse nel telescopic qualche picciolissima stella, 

 vicinissima ad alcuna delle maggiori), for discovering parallax 

 in the fixed stars, f 831 ) Until the middle of the last century, 

 star- catalogues scarcely contained notices of as many as 20 



