PORTION OF THE COSMOS. VISUAL POWER. 49 



great distinctness every evening on the rainless coast of 

 Cumana, and on the high plateaus of the Cordilleras at 

 elevations of twelve thousand feet ; but I have recognised 

 it only rarely and uncertainly in Europe, and in the dry 

 air of the steppes of Northern Asia. The limit within 

 which it is possible, with the naked eye, to separate two 

 objects very near to each other in the heavens, depends, as 

 Madler has very justly remarked, on their relative bright- 

 ness. The two stars of the 3d and 4th magnitudes, marked 

 a Capricorni, which are six and a half minutes apart, are 

 separated without difficulty. Galle thinks it possible, in a 

 very clear atmosphere, to separate with the naked eye e and 

 5 Lyrse, though only three and a half minutes apart, because 

 they are both of the 4th magnitude. 



The too great comparative brightness of the neighbouring 

 planet is also the principal reason why Jupiter's satellites, 

 (one of which, and not all, as is often erroneously stated, 

 is equal in its light to stars of the 5th magnitude), remain 

 invisible to the naked eye. According to recent estimations 

 and comparisons with neighbouring stars by my friend, 

 Dr. Galle, the third satellite, which is the brightest, may 

 correspond to stars from the 5th to the 6th magnitude ; 

 whilst the others correspond, as their light varies, to stars 

 from the 6th to the 7th magnitude. Only occasional in- 

 stances have been cited of persons of extraordinary keenness 

 of vision, (persons who could perceive distinctly with the 

 naked eye stars below the 6th magnitude), having seen 

 any of Jupiter's satellites without a telescope. The angular 

 distance of the third and brightest satellite from the centre 

 of the planet is 4' 42" ; that of the fourth, which is only 

 l-6th smaller than the largest, 8' 16"; and all Jupiter's 



VOL. III. E 



