60 COSMOS. 



test of the power of vision." Notwithstanding the low po- 

 sition of the Great Bear under the tropics, I have very dis- 

 tinctly seen Alcor, evening after evening, with the naked 

 eye, on the rainless shores of Cumana, and on the plateaux 

 of the Cordilleras, which are elevated nearly 13,000 feet 

 above the level of the sea, while I have seen it less frequent- 

 ly and less distinctly in Europe and in the dry atmosphere 

 of the Steppes of Northern Asia. The limits within which 

 the naked eye is unable to separate two very contiguous ob- 

 jects in the heavens depend, as Madler has justly observed, 

 on the relative brilliancy of the stars. The two stars of the 

 third and fourth magnitudes, marked as a Capricorni, which 

 are distant from each other six and a half minutes, can with 

 ease be recognized as separate. Galle thinks that and 6 

 Lyrse, being both stars of the fourth magnitude, may be dis- 

 tinguished in a very clear atmosphere by the naked eye, al- 

 though situated at a distance of only three and a half min- 

 utes from each other. 



The preponderating effect of the rays of the neighboring 

 planet is also the principal cause of Jupiter's satellites re- 

 maining invisible to the naked eye ; they are not all, how- 

 ever, as has frequently been maintained, equal in brightness 

 to stars of the fifth magnitude. My friend, Dr. Galle, has 

 found from recent estimates, and by a comparison with 

 neighboring stars, that the third and brightest satellite is 

 probably of the fifth or sixth magnitude, while the others, 

 which are of various degrees of brightness, are all of the sixth 

 or seventh magnitude. There are only few cases on record 

 in which persons of extraordinarily acute vision that is to 

 say, capable of clearly distinguishing with the naked eye 



image of the neighboring object b is projected. This last imago must 

 therefore either wholly disappear or be dimmed. By day two causes 

 contribute to weaken the light of the stars ; one is the distinct image 



of that portion of the atmosphere included in the direction of the star 

 (the aerial field interposed between the eye and the star), and on which 

 the image of the star is formed, while the other is the light diffused by 

 the dispersion which the defects of the cornea impress on the rays em- 

 anating from all points of the visible atmosphere. At night, the strata 

 of air interposed between the eye and the star to which we direct the 

 instrument, exert no disturbing action ; each star in the firmament forms 

 a more perfect image, but a portion of the light of the stars is dispered 

 in consequence of the imperfect transparency of the cornea. The same 

 reasoning applies to a second, a third, or a thousandth star. The retina, 

 then, is entirely illumined by a diffused light, proportionate to the num- 

 ber of the stars and to their brilliancy. Hence we may imagine that 

 the aggregate of this diffused light must either weaken, or entirely ob- 

 literate the imago of tlie star toward which the eye is directed." 



