[Illustration: Fig. 8.--The Nebula of the Centaur.] On examination of this figure, it will be seen that a nebula is not the result of a collection of stars simply spread out upon a level in space, but of that of an assemblage of stars all placed at unequal distances, and forming almost a sphere. In fact the stars are crowded towards the centre, and are, on the contrary, more and more distant from one another as the outer edge is approached. If a spherical assemblage of stars were observed from a distance, it would present a similar aspect. This leads us to believe that the nebula of the _Centaur_, like the greater number of agglomerations of this kind, is spherical. Is it possible to reckon the stars which form a nebula? Only approximately. Arago estimates the number of stars which form a nebula no larger than the tenth part of the apparent disc of the moon, at twenty thousand, at least. This result may give us an idea of the swarms of suns contained in the nebulæ, for these stellar masses are very numerous in the sky. In the depths of the nebulæ there are luminous points whose nature is as yet unrevealed by the telescope, which cannot be resolved into stars; but analogy leads us to believe that they are other and still more distant nebulæ, which, by reason of their apparent littleness, elude the scope of our instruments. But the time will come, when, thanks to the perfection which our telescopes shall have attained, this theory will be confirmed, and we shall thus see deeper and farther into immensity. The stars which form the nebulæ are sometimes grouped so as to form regular shapes, spheres, or more or less lengthened ellipses. Sometimes the sphere is hollow in the centre, and so forms a ring. Nothing more varied, nothing more strange can be imagined than the forms of those nebulæ which have hitherto been examined, and which already number more than a million, of which no two are precisely alike. Certain nebulæ seem to be double, or joined. Others are lengthened out, like serpents, as in that of the _Shield of Sobieski_, represented in figure 9. Lord Rosse was the first to discover that curious disposition of the nebulæ called _spiral_. Such a form is inexplicable, but it is certain that the suns which compose the nebulæ are often grouped, not around a centre, not in shapeless heaps, but in regular curves, on a system which seems to reveal the existence of some mysterious force acting upon those stars, which are distributed along lines representing spirals of different diameter. [Illustration: Fig. 9.--The Shield of Sobieski.]