An extraordinarily interesting photograph obtained by the celebrated Professor Max Wolf, of Heidelberg, shows us a part of the nebula in the Swan into which a star has penetrated from outside. The intruder has collected about it the nebulous matter it met on its way, and has thus left an empty channel behind it marking its track. Similar spots of vast extent, relatively devoid of nebulous matter, occur very frequently in the irregular nebulæ; they are frequently called “fissures,” or by the specifically English term “rifts,” because they have generally a long-drawn-out appearance. The presumption that these rifts represent the tracks of large celestial bodies which have cut their way through widely expanded nebular masses (Fig. 54) has been entertained for a long time. [Illustration: Fig. 56.—Star cluster in Hercules. Messier 13. Taken at the Yerkes Observatory. Scale, 1 mm. = 9.22 sec. of arc] The smaller and more slowly moving immigrants, on the other hand, are stopped by the particles of the nebulæ. We therefore see the stars more sparsely distributed in the immediate neighborhood of the nebulæ, while in the nebulæ themselves they appear more densely crowded. This fact had struck Herschel in his observations of nebulæ; in recent days it has been investigated by Courvoisier and M. Wolf. In this way several centres of attraction are created in a nebula; they condense the gases surrounding the nebula, and catch, so to say, any stray meteorites and collect them especially in the inner portions of the nebula. We frequently observe, further, how the nebular matter appears attenuated at a certain distance from the luminous bright stars (compare Figs. 52 and 55). Finally, the nebulæ change into star clusters which still retain the characteristic shapes of the nebulæ; of these the spiral is the most usual, while we also meet with conical shapes, originating from conical nebulæ, and spherical shapes (compare Figs. 56, 57, and 58). This is, broadly, the type of evolution through which Herschel, relying upon his observations, presumed a nebula to pass. He was, however, under the impression that the nebulous matter would directly be condensed into star clusters without the aid of strange celestial immigrants. [Illustration: Fig. 57.—Star cluster in Pegasus. Messier 15. Taken at the Yerkes Observatory. Scale, 1 mm. = 6.4 sec. of arc]