370

THE PLANET SATURN.

rings, concentric, or nearly concentric, with one another, thewhole may be divided.

When the rings appear widely opened-ont, important obser-vations may also be made of the shadow of the planet uponthem. For instance, in a drawing by Mr. L. Trouvelot, in theyear 1874, a copy of which is shown in Plate IX., as also in.several published by the Messrs. Bond in Part I. of the beauti-ful and costly second volume of the “Annals of the HarvardCollege Observatory,” various irregularities in the contour ofthe shadow are noticeable. These probably indicate differencesof level in the surface of the rings, which thus distort theoutline where it crosses them. In Plate IX. the edge of theshadow is certainly most remarkably notched as it passes from,the ring A to the ring b. In some of the Harvard drawings acertain amount of shadow is also shown upon the oppositeside of the ball to that upon -which the principal shadow isseen, the cause of which extraordinary phenomenon we arequite unable to explain. All such peculiarities are, however,very important in relation to the physical condition of therings, and the possible existence of an atmosphere belongingto them or to the planet, which may by its refraction producesuch effects. Moreover, near to the ansae certain remarkableirregularities, which are indicated by a series of small notchesin Mr. Trouvelot’s drawing (see Plate IX.), have been fromtime to time observed in the inner edge of the ring A, theoccurrence of which is decidedly puzzling.

In addition to the shadow of the planet upon the rings, thatof the rings upon the planet may be easily seen. Even whenthey altogether vanish from our view, it appears as a narrowblack line across the disc. At other times it may be muchwider, but then it is often to a great degree concealed by therings themselves ; while it is altogether hidden when they areopened out to their utmost extent. So much as we can see,therefore, in general appears as a narrow black line just aboveor below the rings.

Observations near to the time of the disappearance of therings, or when they appear to be extremely thin, are, however,no less interesting than those made when they are wide y