"The important fact clearly brought out at this apparition of _Saturn_ is that the bright rings are not opaque to the light of the sun--and this is really what we should expect from the nature of their constitution as shown by the theory of Clerk Maxwell, and the spectroscopic results of Keeler."[161]
Under certain conditions it would be theoretically possible, according to Mr. Whitmell, to see the globe of Saturn through the Cassini division in the ring. But the observation would be one of great difficulty and delicacy. The effect would be that, of the arc of the division which crosses the planet's disc, "a small portion will appear bright instead of dark, and may almost disappear."[162]
A remarkable white spot was seen on Saturn on June 23, 1903, by Prof. Barnard, and afterwards by Mr. Denning.[163] Another white spot was seen by Denning on July 9 of the same year.[164] From numerous observations of these spots, Denning found a rotation period for the planet of about 10{ h} 39{ m} 21{ s}.[165] From observations of the same spots Signor Comas Sola found a period 10{ h} 38{ m}·4, a close agreement with Denning's result. For Saturn's equator, Prof. Hill found a rotation period of 10{ h} 14{ m} 23{ s}·8, so that--as in the case of Jupiter--the rotation is faster at the equator than in the northern latitudes of the planet. A similar phenomenon is observed in the sun. Mr. Denning's results were fully confirmed by Herr Leo Brenner, and other German astronomers.[166]
Photographs taken by Prof. V. M. Slipher in America show that the spectrum of Saturn is similar to that of Jupiter. None of the bands observed in the planet's spectrum are visible in the spectrum of the rings. This shows that if the rings possess an atmosphere at all, it must be much rarer than that surrounding the ball of the planet. Prof. Slipher says that "none of the absorption bands in the spectrum of _Saturn_ can be identified with those bands due to absorption in the earth's atmosphere," and there is no trace of aqueous vapour.[167]
In September, 1907, M. G. Fournier suspected the existence of a "faint transparent and luminous ring" outside the principal rings of Saturn. He thinks that it may possibly be subject to periodical fluctuations of brightness, sometimes being visible and sometimes not.[168] This dusky ring was again suspected at the Geneva Observatory in October, 1908.[169] M. Schaer found it a difficult object with a 16-inch Cassegrain reflector. Prof. Stromgen at Copenhagen, and Prof. Hartwig at Bamberg, however, failed to see any trace of the supposed ring.[170] It was seen at Greenwich in October, 1908.