Schröter, from different spots, computed periods of 11^h 40^m 30^s, 11^h 51^m, and more than 12^h; but these are probably excessive. The difference of 1^m 37^s between the values of Herschel and Hall is relatively a trivial one, as the markings observed were doubtless atmospheric and subject to irregularities of motion. As to the rotation of the ring, Herschel, in 1789, detected some bright marks on it, and deduced the period as 10^h 32^m 15^s·4[37]. Many astronomical works give the rotation-period of Saturn as 10^h 29^m 16^s·8; and this is adopted in Chambers’s ‘Descriptive Astronomy,’ 4th edit. vol. i. p. 653. The mistake has its origin in Laplace’s _Système du Monde_, where it is stated that Saturn rotates in 0·428 of a day, and the ring in 0·437, which, reduced to hours, minutes, and seconds, give 10^h 16^m 17^s·2 and 10^h 29^m 16^s·8. The equator of Saturn is usually the brightest part of the disk. On its S. side, in recent years, it has been bounded by a very dark narrow belt. Further S. the whole disk seems involved in a faint shading, of a decidedly yellowish hue. Sometimes a considerable number of belts are visible; but they are evidently liable to changes, so that the same number and arrangement are not preserved from year to year. _The Rings._—As to the luminous rings, the extreme diameter of the outer one is about 40″, or more than 170,000 miles; and the black division, separating it from the inner one, is 0″·4, or 1700 miles. The outer ring has a breadth of 2″·4, or 10,000 miles; while the inner one measures 3″·9, or 17,000 miles. The outer ring is less luminous than the inner; the latter, round its outer edges, is extremely brilliant, and has sometimes been described as the brightest part of the Saturnian system. The inner part of this ring is much shaded-off, and offers a strong contrast to the silvery whiteness of the other portion. [Illustration: Fig. 39. Saturn, 1885, Dec. 23, 7^h 54^m. (10-inch reflector, power 252.) ]