_Description of Nebulæ and Clusters of Stars._—The latter objects are included in this chapter for several reasons. In a small telescope nearly all such clusters exhibit the aspect of nebulæ, and they have been catalogued with them, though, as already explained, some great distinctions are to be drawn. To the naked eye the cluster Præsepe, in Cancer, is usually visible as a patch of nebulosity, though on a very clear, dark night stars may be glimpsed sparkling about the spot, and a very small glass will suffice to show it as a nest of stars. This object, and some others of a more difficult character (their component stars being smaller and more compressed), are tabulated (I.) at the end of this chapter. A summary (II.) of globular clusters is also given, together with a list (III.) of nebulæ, a few of which are resolvable into stars[61]. It must be understood that these selections, though comprising many notable objects, are by no means exhaustive, the intention being merely to indicate some typical examples of fine nebulæ and clusters and of peculiarities of form or appearance, such as planetary, annular, elliptical, and centrally condensed nebulæ and loose, compressed, and globular clusters. Some of these objects deserve individual references, as they present interesting details to the telescopic observer and come within the reach of moderate appliances. _Great Nebula in Andromeda_ (M. 31). This object has often been mistaken for a comet, for it is readily perceptible to the eye on a moonless night. It is very large—4° by 2½°, according to Bond, with a 15-inch refractor. He discovered a pair of dark streaks in the brightest region of the nebula, and these may be well seen in a 10-inch reflector. It is really triple; for about 25′ S. of the nucleus there is a very bright, round, resolvable nebula, discovered by Le Gentil, and a third, observed by Caroline Herschel, lies rather further to the N.W. Photographs by Roberts show dark rings dividing the bright interior parts of the nebula from the outer, and imparting to it a decided spiral tendency. This nebula has hitherto resisted attempts to resolve it into stars, though many hundreds have been seen in the foreground. But its spectrum is continuous, so that its stellar character is to be inferred.