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THE PLANET SATURN.

thought to indicate the existence of some kind of atmosphereover that portion of the rings. It is, however, more probablyproduced by the dusky or crape-like ring, c; which, if so, maybe considerably thicker than a and B. This is well shown—seeFig . LXXXVI.—in a drawing by Mr. Wray, dated December26th, 1861, in which two of the satellites are also visible,apparently resting upon the line of the outer rings.

It is generally found that the above-mentioned fine line oflight, before it altogether disappears, breaks up into severalmore or less isolated portions. An ingenious suggestion hasbeen made by the Messrs. Bond,* that this may be simply theresult of an optical effect produced by the inner and outeredges of the various rings being seen nearly edgewise, so thattheir light is united in certain parts, where it is consequentlystrong enough to be visible; while in other parts, where thebright edge of only one is turned towards an observer, theamount of light is too small to be perceived.

There are also some other classes of observations of greatimportance which may be made at such times. For instance,the line of light upon one side of the planet becomes some-times so fine as to be invisible in telescopes of moderatepower some days before this is the case upon the oppositeside. This, of course, goes against the supposition of a rotationof the rings round Saturn ; for, if they so revolved, we shouldexpect that the more noticeable, or thicker, portion would goregularly round from one side to the other of the disc. It hasno doubt been thought that certain observations of the vanish-ing rings by Sir W. Herschel, and by one or two other observers,have given indications of such a rotation, at least in one ofthem. We consider, however, and we shall presently endeavourto prove, that any such movement of any ring as a whole musthave been apparent rather than real.

Very careful measurements indicate that the centre of theplanet is not placed quite in the centre of the rings. If this

* A copy of the original diagram by which it was illustrated in vol. ii.of the “Annals of the Harvard College Observatory” may be seen inChambers’ “Handbook of Descriptive Astronomy,’’ and in Guillemin’s“ The Heavens.”