A GIANT PLANET. 297 



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 1 

 had looked at Jupiter without knowing any thing of the positions of 

 his satellites, I should have said that a shadow (of a satellite) was pass- 

 ing. I remember having seen the like phenomenon many years ago ; 

 but my impression is that I had never seen the disk 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 

 the exceeding brilliancy of the surface of the planet. In the same way, 

 the solar spots, if not surrounded by the marvellous splendor of the 

 sun's surface, would doubtless appear as brilliant objects." 



Next let us hear Secchi's account. " On the evening of February 

 3d," he says, " I observed the transit of the third satellite and that of 

 its shadow. The satellite seemed almost- black when it was upon the 

 middle of the planet's disk, and notably smaller than its shadow, which 

 was visible at the same time ; one would have estimated it at only one- 

 half. In approaching the edge the satellite disappeared, and reappeared 

 soon after, close by the edge, but as a bright point. This fact is not a 

 new one for the other satellites, but for the third it is unique. This re- 

 sult shows also the great difference of luminosity at the centre and near 

 the edge of the planet, a difference already confirmed by photogra- 

 phy." 



It is hardly necessary to point out how strikingly these facts illus- 

 trate and confirm Dr. Zollner's observations. But they also supply 

 fresh evidence of a very interesting nature. 



Although a part of the difference dwelt on in Secchi's closing words 

 may be ascribed to the oblique incidence of the light near the planet's 

 edge, yet it does not appear to us that the whole difference can be thus 

 explained. A difference so great that a satellite appears as a bright 

 point close by the planet's edge, and almost black near the middle of 

 the disk, suggests that the light near the edge is not reenforced by the 

 inherent luminosity of our theory, that luminosity adding only to the 

 brightness of the central parts of the disk. We would not insist too 

 strongly on this inference, because the darkening due to oblique inci- 

 dence is, under certain circumstances, very obvious to direct observa- 

 tion. But it seems to us that a portion of the difference should be re- 

 ferred to the inherent luminosity of the central parts of the disk. 

 This being admitted, it would follow that the real solid globe of the 

 planet is much smaller than the globe measured by astronomers ; and 

 that, therefore, instead of that amazingly small density which is so 

 perplexing a feature of the planet's physical condition, Jupiter's globe 

 may have a density equalling or exceeding that of the earth. 



And, after ail, let us remember that the theory that Jupiter is an 

 intensely heated globe a theory to which we have been led by the 

 consideration of many observed facts, and which in its turn suggests 

 very satisfactory explanations of other observed facts would merely 



