106 THE BORDERLAND OF SCIENCE. 



his satellites pass across his face, and nothing can be 

 easier than to observe whether they appear darker or 

 brighter than his surface. 



It was an observation such as this which Mr. Lassell 

 had made on the night when he noticed the ruddiness 

 of Jupiter's great medial belt. By a singular chance 

 Father Secchi made a similar observation during his 

 researches, and the reader will see, when we have quoted 

 the narratives of both these observers, that the com- 

 parative darkness of all four satellites will have been 

 established. c The fourth satellite,' says Lassell, < has 

 begun again for a season to cross the planet's disc, and 

 I have looked out for opportunities of observing its 

 passages, and was favoured on the night of the 30th 

 December last by witnessing a part of its passage under 

 circumstances more than usually propitious. On its 

 first entrance it was scarcely to be distinguished from 

 the edge, not appearing at all as the others do, as a 

 round bright spot. As it advanced it grew gradually 

 manifestly darker than the surface of the planet, and 

 by the time it had advanced a fourth of the way across 

 it had become a very dark if not a black spot so dark, 

 indeed, that if I had looked at Jupiter without knowing 

 anything of the positions of his satellites, I should 

 have said that a shadow (of a satellite) was passing. I 

 remember having seen the like phenomenon many years 

 ago ; but my impression is that I had never seen the 

 disc of the satellite so near to absolute blackness before. 

 Of course it is only by contrast that it can possibly so 

 appear ; and we have in this fact a striking proof of 



