The most of these little moons are, of course, beyond the power of small glasses; but a 2-inch will show Titan perfectly well. Japetus also is not a difficult object, but is much easier at his western than at his eastern elongation, a fact which probably points to a surface of unequal reflective power. Rhea, Dione, and Tethys are much more difficult. Kitchiner states that a friend of his saw them with 2-7/10-inch aperture, the planet being hidden; but probably his friend had been amusing himself at the quaint old gentleman's expense. Noble concludes that with a first-class 3-inch and under favourable circumstances four, or as a bare possibility even five, satellites may be seen; and I have repeatedly seen all the five with 3-7/8-inches. The only particular advantages of seeing them are the test which they afford of the instrument used, and the accompanying practice of the eye in picking up minute points of light. There is a considerable interest in watching the gradual disappearance of the brilliant disc of Saturn behind the edge of the field, or of the thick wire which may be placed in the eye-piece to hide the planet, and then catching the sudden flash up of the tiny dots of light which were previously lost in the glare of the larger body. For purposes of identification, recourse must be had to the 'Companion to the Observatory,' which prints lists of the elongations of the various satellites and a diagram of their orbits which renders it an easy matter to identify any particular satellite seen. Transits are, with the exception of that of Titan, beyond the powers of such instruments as we are contemplating. The shadow of Titan has, however, been seen in transit with a telescope of only 2-7/8-inch aperture. [Footnote *: The plane of the rings passes through the earth on April 13, and through the sun on July 27, 1907, at which periods it is probable that the rings will altogether disappear.] CHAPTER XII URANUS AND NEPTUNE Hitherto we have been dealing with bodies which, from time immemorial, have been known to man as planets. There must have been a period when one by one the various members of our system known to the ancients were discriminated from the fixed stars by unknown but patient and skilful observers; but, from the dawn of historical astronomy, up to the night of March 13, 1781, there had been no addition to the number of those five primary planets the story of whose discovery is lost in the mists of antiquity. It may be questioned whether any one man, Kepler and Newton being possible exceptions, has ever done so much for the science of astronomy as was accomplished by Sir William Herschel. Certainly no single observer has ever done so much, or, which is almost more important than the actual amount of his achievement, has so completely revolutionized methods and ideas in observing.