We had occasion to quote a passage from Mr Mill, in which he remarks that the majority of scientific men seem not yet to have completely got over the difficulty of conceiving matter to act (contrary to the old maxim) where it is not; "for though," he says, "they have at last learned to conceive the sun _attracting_ the earth without any intervening fluid, they cannot yet conceive the sun _illuminating_ the earth without some such medium." But it is not only this difficulty (which doubtless, however, is felt) of conceiving the sun illuminating the earth without any medium by which to communicate its influence, which leads to the construction of the hypothesis, either of an undulating ether, or of emitted particles. The analogy of the other senses conducts us almost irresistibly to the imagination of some such medium. The nerves of sense are, apparently, in all cases that we can satisfactorily investigate, affected by contact, by impulse. The nerve of sight itself, we know, when touched or pressed upon, gives out the sensation of light. These reasons, in the first place, conduct us to the supposition of some medium, having immediate communication with the eye; which medium, though we are far from saying that its existence is established, is rendered probable by the explanation it affords of optical phenomena. At the same time it is evident that the hypothesis of an undulating ether, assumes a fluid or some medium, the existence of which cannot be directly ascertained. Thus stands the hypothesis of a luminiferous ether--in what must be allowed to be a very unsatisfactory condition. But a condition, we think, very superior to the astronomical speculation of Laplace, which Mr Mill, after scrutinizing the preceding hypothesis with the utmost strictness, is disposed to treat with singular indulgence. "The speculation is," we may as well quote throughout Mr Mill's words, "that the atmosphere of the sun originally extended to the present limits of the solar system: from which, by the process of cooling, it has contracted to its present dimensions; and since, by the general principles of mechanics, the rotation of the sun and its accompanying atmosphere must increase as rapidly as its volume diminishes, the increased centrifugal force generated by the more rapid rotation, overbalancing the action of gravitation, would cause the sun to abandon successive rings of vaporous matter, which are supposed to have condensed by cooling, and to have become our planets.