Keeler's photography of the nebulæ led him to open another chapter in nebular investigation with the startling discovery that "most of the nebulæ have the spiral structure." This applied not only to the faint nebulæ which he discovered, but to the nebulæ already known. Keeler's successors have confirmed this discovery: it is certain that the great majority of the nebulæ have the spiral form. What the relative number of spirals and formless nebulæ may be remains for the future to decide.
The spirals vary all the way from the great. Andromeda nebula down to those so small that the photographic plate is just able to separate the details of structure; and there is no reason to doubt that more powerful instruments would show still smaller objects to have the spiral structure.
There are irregular nebulæ of all sizes. The brilliant Orion nebula is diminutive in size compared with the faint nebulosity, discovered by William H. Pickering in 1889, which forms the background for almost the whole of the constellation of Orion. The well-known nebulous structure connected with the brighter Pleiades stars is small in comparison with the area covered by a faint exterior nebulosity discovered by Barnard in 1893. There are very great irregular nebulæ such as the Network Nebula in Cygnus and the nebulous background in the Greater Magellanic Cloud. Barnard's wonderful photographs of the Milky Way have recorded many extensive nebulous fields, especially in regions where the background of the galaxy shows relatively few stars (see Fig. 2).
The so-called planetary nebulæ are of special interest, as we shall learn in the sequel. Small in size, all more or less dense, some quite
