Thus far we get, and no farther. Our solar system appears to lie somewhere near the centre of a great collection of stars, separated each one from the other, on an average, by some 40 billions of miles; the whole being arranged in the form of a mighty globular cluster. Light from the nearest of these stars takes some four years to come to us. It takes about 1000 times as long to reach us from the confines of the system. This globe of stars is wrapt around closely by a stellar girdle, the individual stars in which are set together more densely than those in the globe itself. The entire arrangement appears to be constructed upon a very regular plan. Here and there, as Professor Newcomb points out, the aspect of the heavens differs in small detail; but generally it may be laid down that the opposite portions of the sky, whether in the Milky Way itself, or in those regions distant from it, show a marked degree of symmetry. The proper motions of stars in corresponding portions of the sky reveal the same kind of harmony, a harmony which may even be extended to the various colours of the stars. The stellar system, which we see disposed all around us, appears in fine to bear all the marks of an _organised whole_. The older astronomers, to take Sir William Herschel as an example, supposed some of the nebulæ to be distant "universes." Sir William was led to this conclusion by the idea he had formed that, when his telescopes failed to show the separate stars of which he imagined these objects to be composed, he must put down the failure to their stupendous distance from us. For instance, he thought the Orion Nebula, which is now known to be made up of glowing gas, to be an external stellar system. Later on, however, he changed his mind upon this point, and came to the conclusion that "shining fluid" would better account both for this nebula, and for others which his telescopes had failed to separate into component stars. The old ideas with regard to external systems and distant universes have been shelved as a consequence of recent research. All known clusters and nebulæ are now firmly believed to lie _within_ our stellar system. This view of the universe of stars as a sort of island in the immensities, does not, however, give us the least idea about the actual extent of space itself. Whether what is called space is really infinite, that is to say, stretches out unendingly in every direction, or whether it has eventually a boundary somewhere, are alike questions which the human mind seems utterly unable to picture to itself. [35] The Ptolemaic idea dies hard! [36] Even the Milky Way itself is far from being a blaze of light, which shows that the stars composing it do not extend outwards indefinitely.