_Groups of Stars._—Great dissimilarity is apparent in the clustering of stars. The heavens furnish us with all gradations—from the loose, open groups like that in Coma Berenices, in the Pleiades, or in Præsepe, to the compact globular clusters, in which some thousands of stars are so densely congregated that considerable optical power is required to disintegrate them. Some, it is true, yield more easily than others. The great cluster (Messier 13) in Hercules readily displays the swarms of stars of which it is composed; but others are so difficult that it is only in the largest instruments they are resolved into star-dust. Further references to these wonderful objects will be made in the next chapter, and some of the principal examples described; our purpose here is to allude to a few of the more scattered groups, and to some noteworthy instances of multiple stars. _Coma Berenices._ A naked-eye cluster, consisting of many stars, chiefly from the 5th to 6th mags. A telescope adds a number of smaller stars. Nebulæ may be often swept up hereabouts, as it is not far north of the rich nebulous region of Virgo. _The Pleiades._ Six stars are usually distinguished by the naked eye, and a seventh is occasionally remarked. Möstlin (the instructor of Kepler) counted fourteen, Miss Airy has drawn twelve, and Carrington, like Möstlin, saw fourteen. In 1877 I distinctly made out fourteen stars in this group. The telescope reveals a considerable number of small stars and Tempel’s large nebula near Merope. Kepler saw thirty-two stars with a telescope, and Hooke seventy-eight; but Wolf, at Paris, after three years of unremitting labour with a 4-foot reflector, catalogued 671 stars in the group. A photograph, however, with a 12-inch refractor showed 1421 stars; and a more recent negative includes no less than 2326. There is an interesting little triangle close to the brightest star, Alcyone; and several of the leading stars are involved in nebulosity, discovered by means of photography. _Præsepe._ A fine group of small stars, divisible by the unaided eye on a clear night. Chambers says the components are not visible without a telescope; while Webb notes that the group is just resolvable by the naked eye. Thirty-six stars were glimpsed with Galilei’s telescope; but modern instruments show many more. Marth, using Lassell’s 4-foot reflector at Malta, discovered several faint nebulæ and nebulous stars in this cluster.