For there are different kinds of nebulæ. Some may, as just said, be vast gatherings of stars lying at distances beyond calculation, almost beyond imagination. Others appear rather to be clusters of stars, like those already described, probably situated in our own galaxy of stars. There is also a third kind of nebula. Many nebulæ, once supposed to be clusters of stars, having been lately examined by means of the spectroscope, are found to be enormous masses of glowing gas, and not solid bodies at all. The number of nebulæ known amounts to many thousands. They are commonly divided into classes, according to their seeming shape. There are nebulæ of regular form, and nebulæ of irregular form. There are circular nebulæ, oval nebulæ, annular nebulæ, conical nebulæ, cometary nebulæ, spiral nebulæ, and nebulæ of every imaginable description. These shapes would no doubt entirely change, if we could see them nearer; and indeed, even in more powerful telescopes, they are often found to look quite different. While on the subject of clusters and nebulæ, mention should be made of the famous Magellanic Clouds in the southern heavens. Sometimes they are called the Cape Clouds. They differ from other nebulæ in many points, and more particularly in their apparent size. The Great Cloud is about two hundred times the size of the full moon, while the Small Cloud is about one-quarter as large. In appearance they are not unlike two patches of the Milky Way, separated and moved to a distance from the main stream. These clouds are surrounded by a very barren portion of the heavens, containing few stars; but in themselves they are peculiarly rich. Seen through a powerful telescope, they are found to abound with stars. The Greater Cloud alone contains over six hundred from the seventh to the tenth magnitudes, countless tiny star-points of lesser magnitudes, star-clusters of all descriptions, and nearly three hundred nebulæ,--all crowded into this seemingly limited space. Among many famous nebulæ, the one in Orion and the one in Andromeda may be particularly mentioned. The size of the latter, as compared with the size of our Solar System, is shown in the cut of this nebula. On the hypothesis of a complete resolvability into stars, the mind is lost in numbering the myriads of suns, the agglomerated individual lights of which produce these nebulous fringes of such different intensities. What must be the extent of this universe, of which each sun is no more than a grain of luminous dust! CHAPTER XXVI. THE PROBLEM OF SUN AND STAR DISTANCES. One question had long occupied the minds of scientific men without the finding of any satisfactory answer, and this was the distance of the sun from our earth.