64 COSMOS. 



atmosphere of the Steppes of Northern Asia. The limits 

 within which the naked eye is unable to separate two very 

 contiguous objects in the heavens depend, as Madler has 

 justly observed, on the relative brilliancy of the stars. The 

 two stars of the 3rd and 4th magnitudes, marked as a Capri- 

 corni, which are distant from each other six- and-a-half minutes, 

 can with ease be recognized as separate. Galle thinks that 

 f and 5 Lyrae, being both stars of the 4th magnitude, may be 

 distinguished in a very clear atmosphere by the naked eye, 

 although situated at a distance of only three-and-a-half minutes 

 from each other. 



The preponderating effect of the rays of the neighbouring 

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

 ing invisible to the naked eye ; they are not all, however, as 

 has frequently been maintained, equal in brightness to stars of 

 the 5th magnitude. My friend, Dr. Galle, has found from 

 recent estimates, and by a comparison with neighbouring 

 stars, that the third and brightest satellite is probably of the 

 5th or 6th magnitude, whilst the others, which are of various 

 degrees of brightness, are all of the 6th or 7th magnitude. 

 There are only few cases on record in which persons of ex- 

 traordinarily acute vision that is to say, capable of clearly 

 distinguishing with the naked eye stars fainter than those of 

 the 6th magnitude, have been able to distinguish the satellites 

 of Jupiter 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 th smaller than the 

 largest is 8' 16" : and all Jupiter's satellites sometimes exhibit, 

 as Arago maintains, 16 a more intense light for equal surfaces 



16 Arago, in the Annuaire pour 1842, p. 284, and in the 

 Comptes rendus, torn. xv. 1842, p. 750. (Schum. Astron. 

 JVcAr.,no. 702.) "I have instituted some calculations of mag- 

 nitudes, in reference to your conjectures on the visibility 

 of Jupiter's satellites," writes Dr. Galle, in letters addressed 



