The Saturnian System. [January, 
which exists in the case of our own earth, seems to be the 
explanation to which we are driven by the facts of the case, 
and, from what we know of the sun, may be regarded as 
a sufficient explanation. 
It remains to be seen, however, whether this explanation 
is supported or negatived by other facts. If it be the correct 
explanation, moreover, not only should it be in accordance 
with the facts hitherto considered, but it should indicate the 
existence of other relations than those which have led us to 
it. In other words, on carefully considering this theory, 
consequences should appear to follow from it which corre- 
spond with the actual results of observation. 
Let us begin, however, by considering the arguments which 
at a first view seem to oppose the theory that the globes of 
Saturn and Jupiter are intensely heated. 
It is manifest that the heat required by the theory is such 
that the substance of the real globes of Saturn and Jupiter 
must be self-luminous, and perhaps to a high degree. Now 
it is certain that the discs of these planets, as we see them, 
do not owe their light chiefly to inherent luminosity. This 
is shown, not only by what I shall mention presently as to 
the quantity of light received from these planets, but also 
by a circumstance which deserves more careful attention 
than it has yet, I think, received. Not only does the disc 
grow darker near the edge,* as it would if the light were 
reflected light, but both Jupiter and Saturn, when near their 
quadratures, show a marked defalcation of light on the side 
where lies the terminator of the really gibbous disc. Again, 
we see that the satellites of Jupiter disappear in the shadow 
of their primary, which would not be the case if the planet 
shone with a very strong inherent light. An even clearer 
argument at a first view is found in the circumstance that 
the shadows of the satellites look black, as though the whole 
of Jupiter’s light were reflected sunlight. 
None of these circumstances, however, suffices to prove 
that the disc of Jupiter does not possess some degree, and 
even it may well be a considerable degree, of inherent 
luminosity. It is manifest that the vanishing of the satel- 
lites in Jupiter’s shadow would only imply that his disc 
does not glow with an intense lustre of its own, a fa¢ét which 
is otherwise obvious. The darkening at the edge of the 
* To ordinary vision, the reverse appears the case; but the reason of this is 
that the dark sky on which, as on a background, the disc is seen, causes the 
edge of the disc to look dark by an effec of contrast. Mr. Browning tested 
the matter two or three years ago with a graduated darkening-glass, and found 
that, as had been theoretically inferred from the aspect of the satellites on 
different parts of the disc, the edge is darker than the middle. 
