286 ON THÉ LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
ground, is an error; for since they become relatively darker than the planet only when 
at a considerable distance within the limb, it is plain that a constant period of rotation 
of the satellite on its axis will not account for its always becoming white when near 
its egress, so as to present precisely the same aspect that it has just after its ingress, 
whether the chord described over the disc of Jupiter be large or small. This alone is 
a sufficient objection to the proposed explanation; it is, moreover, inconsistent with 
the change of aspect which the satellites must present to the Earth when in conjunc- 
tion with the primary, in consequence of the revolution of Jupiter about the Sun. 
There is no way, in fact, of accounting for the phenomenon in question, except by ` 
supposing that the central regions of Jupiter are so much brighter than the parts 
towards the circumference, that an object, of an intensity intermediate between the two, 
is made, by the mere effect of contrast, to appear white when projected upon the lat- 
ter, and dark when projected upon the former. This implies a disproportion greater 
than 1 : 6 between the whiteness of the satellite and of the bright belts of Jupiter, as 
I have found by experiments similar to those related above. 
This defect of brightness near the limb, as has been before remarked, authorizes 
the application to this planet of Lambert's principle of opaque reflection, and 
scarcely leaves us at liberty to assume that a much larger proportion of the whole 
light reflected is directed towards the Earth than that principle allows. Perhaps, 
however, by admitting a tendency in this direction, and remembering that a re- 
flective capacity equal to that of the whitest substances known to us probably belongs 
to the sunward side of the clouds in our atmosphere, and not less so to the cloud- 
mantle in which Jupiter is enveloped, we may have a more acceptable explanation of 
its remarkable brilliancy than to suppose it to be in any degree selfluminous. If 
Venus, like Jupiter, be surrounded by a dense envelope of clouds, — a. supposition 
otherwise quite in keeping with its physical aspect, — the difficulty of accounting for 
its high albedo would be considerably lessened. Its excess of light when in the 
half of its orbit nearest the Earth would then be merely an example of the phe- 
nomenon presented by the dazzling whiteness of clouds in our atmosphere when seen 
near the Sun. : 
Erratum. — On p. 225, for S = 477530, read S = 470980. 
