The field of discovery always appears to be completely occupied until the advent of a great man, who, even by his way of putting old and familiar facts, shows the paths along which discoveries must come, if at all. This faculty comes from profound reflection on the nature of the subject itself, from a sort of transmuting power which changes the words of the books into the things of reality. HERSCHEL'S paper on _Saturn_, in 1790, is an admirable example of this.
HERSCHEL'S observations on _Saturn_ began in 1772. From 1790 to 1808 he published six memoirs on the figure, the ring, and the satellites of this planet. The spheroidal shape of the ball was first discovered by him, and we owe much of our certain knowledge of the constitution of the rings to his work. The sixth and seventh satellites, _Mimas_ and _Enceladus_, were discovered by him in 1789. The periods of rotation of the ball and of the ring were also fixed. In his conclusions as to the real figure of the rings, there is a degree of scientific caution which is truly remarkable, and which to-day seems almost excessive.
In his paper of 1792, HERSCHEL shows that the most distant satellite of _Saturn_--_Japetus_--turns once on its axis in each revolution about its primary, just as our moon does. He says of this:
"I cannot help reflecting with some pleasure on the discovery of an analogy which shows that a certain uniform plan is carried on among the secondary planets of our solar system; and we may conjecture that probably most of the satellites are governed by the same law; especially if it be founded on such a construction of their figure as makes them more ponderous towards their primary planets."
I believe the last suggestion to have been the first statement of the possible arrangement of matter in satellites, which was afterwards so forcibly maintained by HANSEN in his theory of the moon. HANSEN'S researches show the consequences of such an arrangement, although they do not prove its existence.
It should be recorded that the explanation which is to-day received of the belts and bands upon _Jupiter_, is, I believe, first found in HERSCHEL'S memoir on _Venus_ (1793). His memoir of 1797, on the changeable brightness of the satellites of _Jupiter_, has already been referred to. The times of the rotation of the satellites on their axes was first determined by HERSCHEL from these observations, which also contain accounts of the curious, and as yet unexplained, phenomena attending their appearances on the disc of the planet.