And yet again beside the stars the telescope reveals {89} to us the NEBULÆ. Some of these are the Irregular Nebulæ--wide-stretching, cloudy, diffused masses of filmy light, like the Great Nebula in Orion. Others are faint but more defined objects, some of them with small circular discs, and looking like a very dim Uranus, or even like Saturn--that is to say, like a planet with a ring round its equator. This class are therefore known as "+Planetary Nebulæ+," and, when bright enough to show traces of colour, appear green or greenish blue. These are, however, comparatively rare. Other of these faint, filmy objects are known as the "+White Nebulæ+," and are now counted by thousands. They affect the spiral form. Sometimes the spiral is seen fully presented; sometimes it is seen edgewise; sometimes more or less foreshortened, but in general the spiral character can be detected. And these White Nebulæ appear to shun the Galaxy as much as the Planetary Nebula; and Star Clusters prefer it; indeed the part of the northern heavens most remote from the Milky Way is simply crowded with them. It can be by no accident or chance that in the vast edifice of the heavens objects of certain classes should crowd into the belt of the Milky Way, and other classes avoid it; it points to the whole forming a single growth, an essential unity. For there is but one belt in the heavens, like the Milky Way, a belt in which small stars, New Stars, and Planetary Nebulæ find their favourite home; and that belt encircles the entire heavens; and similarly that belt is the only region from which the White Nebulæ appear to be repelled. The Milky Way forms the foundation, the strong and buttressed wall of the celestial building; the White Nebulæ close in the roof of its dome. {90} And how vast may that structure be--how far is it from wall to wall? That, as yet, we can only guess. But the stars whose distances we can measure, the stars whose drifting we can watch, almost infinitely distant as they are, carry us but a small part of the way. Still, from little hints gathered here and there, we are able to guess that, though the nearest star to us is nearly 300,000 times as far as the Sun, yet we must overpass the distance of that star 1000 times before we shall have reached the further confines of the Galaxy. Nor is the end in sight even there.