sphere of luminous vapor; and the planets, plunged into this atmosphere, may, instead of describing regular orbits, plough their way in spiral paths through the nebulous abyss to its central nucleus. 8. Clustered stars, then, and double stars, appear to give us but little promise of inhabitants. We must next turn our attention to the single stars, as the most hopeful cases. Indeed, it is certain that no one would have thought of regarding the individual stars of clusters, or of pairs, as the centres of planetary systems, if the view of insulated stars, as the centres of such systems, had not already become familiar, and, we may say, established. What, then, is the probability of that view? Is there good evidence that the Fixed Stars, or some of them, really have planets revolving round them? What is the kind of proof which we have of this? 9. To this we must reply, that the only proof that the fixed stars are the centres of planetary systems, resides in the assumption that those stars are _like the Sun_;--resemble him in their qualities and nature, and therefore, it is inferred, must have the same offices, and the same appendages. They are, as the Sun is, independent sources of light, and thence, probably, of heat; and therefore they must have attendant planets, to which they can impart their light and heat; and these planets must have inhabitants, who live under and enjoy those influences. This is, probably, the kind of reasoning on which those rely, who regard the fixed stars as so many worlds, or centres of families of worlds. 10. Everything in this argument, therefore, depends upon this: that the Stars are _like the Sun_; and we must consider, what evidence we have of the exactness of this likeness. 11. The Stars are like the Sun in this, that they shine with an independent light, not with a borrowed light, as the planets shine. In this, however, the stars resemble, not only the Sun, but the nebulous patches in the sky, and the tails of comets; for these also, in all probability, shine with an original light. Probably it will hardly be urged that we see, by the very appearance of the stars, that they are of the nature of the Sun: for the appearance of luminaries in the sky is so far from enabling us to discriminate the nature of their light, that to a common eye, a planet and a fixed star appear alike as stars. There is no obvious distinction between the original light of the stars and the reflected light of the planets. The stars, then, being like the sun in being luminous, does it follow that they are, like the sun, definite dense masses?[8] Or are they, or many of them, luminous masses in a far more diffused state; visually contracted to points, by the immense distance from us at which they are?