An interesting discovery of recent date, made by Professor Bailey in photographing star clusters, is that some few of them, which are especially rich in stars, contain an extraordinary number of variable stars, mostly very faint and of short period. Two clusters, one in the northern and one in the southern hemisphere, contain each more than a hundred variables, and an even more extraordinary case is presented by a cluster, called Messier 5, not far from the star α Serpentis, which contains no less than sixty-three variables, all about of the fourteenth magnitude, all having light periods which differ but little from half a day, all having light curves of about the same shape, and all having a range of brightness from maximum to minimum of about one magnitude. An extraordinary set of coincidences which "points unmistakably to a common origin and cause of variability." [Illustration: FIG. 136.--Star cluster in Hercules.--KEELER.] [Illustration: FIG. 137.--The Andromeda nebula as seen in a very small telescope.] [Illustration: FIG. 138.--The Andromeda nebula and Holmes's comet. Photographed by BARNARD.] [Illustration: FIG. 139.--A drawing of the Andromeda nebula.] [Illustration: FIG. 140.--A photograph of the Andromeda nebula.--ROBERTS.] 214. NEBULÆ.--Returning to Fig. 136, we note that its background has a hazy appearance, and that at its center the stars can no longer be distinguished, but blend one with another so as to appear like a bright cloud. The outer part of the cluster is _resolved_ into stars, while in the picture the inner portion is not so resolved, although in the original photographic plate the individual stars can be distinguished to the very center of the cluster. In many cases, however, this is not possible, and we have an _irresolvable cluster_ which it is customary to call a _nebula_ (Latin, _little cloud_). The most conspicuous example of this in the northern heavens is the great nebula in Andromeda (R. A. 0^{h} 37^{m}, Dec. + 41°), which may be seen with the naked eye as a faint patch of foggy light. Look for it. This appears in an opera glass or very small telescope not unlike Fig. 137, which is reproduced from a sketch. Fig. 138 is from a photograph of the same object showing essentially the same shape as in the preceding figure, but bringing out more detail. Note the two small nebulæ adjoining the large one, and at the bottom of the picture an object which might easily be taken for another nebula but which is in fact a tailless comet that chanced to be passing that part of the sky when the picture was taken. Fig. 139 is from another drawing of this nebula, although it is hardly to be recognized as a representation of the same thing; but its characteristic feature, the two dark streaks near the center of the picture, is justified in part by Fig. 140, which is from a photograph made with a large reflecting telescope. [Illustration: FIG. 141.--Types of nebulæ.]