This spiral form (Figs. 47 and 48) of the outer portions of the nebulæ has for a long time excited the greatest attention. In almost all the investigated instances it has been observed that two spiral branches are coiling about the central body. This would indicate that the matter is in a revolving movement about the central axis of the spiral, and that it has streamed away from the axis in two opposite directions. Sometimes the matter appears arranged as in a coil; of this type the great nebula of Andromeda is the best-known example (Fig. 49). A closer inspection of this nebula with more powerful instruments indicates, however, that it is also spiral and that it appears coiled, because we are looking at it from the side. The late famous American astronomer Keeler, who has studied these nebulæ with greater success than any one else, has catalogued a great many of them in all the divisions of the heavens which were accessible to his instruments, and he has found that these formations are predominatingly of a spiral nature. [Illustration: Fig. 51.—Central portion of the great nebula in Orion. Taken at the Yerkes Observatory. Scale, 1 mm. = 12 sec. of arc] Some nebulæ, like the so-called planetary nebulæ, offer rather the appearance of luminous spheres. We may assume in these cases that the explosions were less violent, and that the spirals, therefore, are situated so closely together that they seem to merge into one another. Possibly the inequalities in their development have become equalized in the course of time. A few nebulæ are ring-shaped, as the well-known nebula of Lyra (see Fig. 50). These rings may, again, have been formed out of spiral nebulæ, and the spirals may have gradually been obliterated by rotation, while the central nebulous matter may have been concentrated on the planets travelling round the central star. Schaeberle, an eminent American astronomer, has discovered traces of spiral shape also in the Lyra nebula. Another kind of nebula is the ordinary nebula of vast extension and irregular shape, evidently formed out of most extremely attenuated matter; well-known characteristic examples are found in Orion, about the Pleiades, and in the Swan (Figs. 51, 52, and 53). In these nebulæ portions of a spiral structure have likewise often been discerned. [Illustration: Fig. 52.—Nebular striæ in the stars of the Pleiades. Taken at the Yerkes Observatory on October 19, 1901. Scale, 1 mm. = 42.2 sec. of arc]