it with the principal mass, the whole object resembles a pair of capital Greek omegas connected at their bases. In ordinary telescopes the outline resembles that of a swan minus its legs! : 11 is a curious object near the 5th-magnitude star 14 Vulpeculæ ; it is shaped like a double-headed shot, or dumb-bell, and is usually known as the "Dumb-bell" nebula. In a small telescope it appears like two roundish nebulosities, in contact the one with the other, or nearly so. Sir J. Herschel saw it with "an elliptical outline of faint light enclosing the two chief masses," but Lord Rosse's reflectors materially change the appearance of the object his 3 ft. reflector destroys the regular elliptic outline seen by Sir J. Herschel, and his 6 ft. instrument resolves it into stars, making the general outline to resemble that of a chemical retort. The history of the observation of this nebula affords a striking illustration of the fallacy associated with the happily now exploded Nebular hypothesis. 12. "A most wonderful phenomenon. A very large space, 20 or 30' broad in P. D. and 1m or 2m in R. A., full of nebula and stars mixed. The nebula is decidedly attached to the stars, and is as decidedly not stellar. It forms irregular lacework marked out by stars, but some parts are decidedly nebulous, wherein no stars can be seen." Little credit must be attached to the italicised clause in this quotation. In the southern hemisphere, and not far from the Pole, are the Magellanic clouds, or nubeculæ major and minor, so called from their cloud-like appearance. The former is situated in the constellation Dorado, and the latter in Toucan. They are of a somewhat oval shape, and are both visible to the naked eye when the Moon is not shining; but the smaller disappears in strong moonlight. Sir J. Herschel, when at the Cape, examined these remarkable objects with his large telescope, and describes them as consisting of swarms of stars, clusters, and nebula of every description. The larger one covers an area of about 42 square degrees, and the smaller of 10 square degrees. The nebulæ are very far from being uniformly distributed in the heavens, but congregate especially in a zone crossing at right angles the Milky Way. They are exceedingly abundant in the constellation Virgo. Sir J. Herschel's Catalogue of 1864 contains 5079 of these objects, which are thus distributed through On the distribution of the nebulæ, Guillemin remarks as follows: "This is very unequal in the northern hemisphere, and in those parts of the southern one visible in the northern temperate zone. The greatest number is found in a zone which scarcely embraces the eighth part of the heavens. The constellations Leo, Ursa Major, Camelopardus, Draco, Boötes, Coma Berenicis, and Canes Venatici, but principally Virgo, form this zone, which extends as far as the middle of Centaurus: it is known under the name of the nebulous region of Virgo. Nearly at the opposite pole of the sky, another agglomeration of nebulæ embraces Andromeda, Pegasus, and Pisces, and extends lower than the firstnamed constellation into the southern heavens. "It is noteworthy that the regions nearest the Milky Way are the poorest in nebula, whilst the two richest regions lie at the two poles of that great belt in which the stars are so numerous and condensed. The nebulæ are more uniformly spread over the zone which surrounds the South Pole; they are at the same time much less numerous. On the other hand, there are two magnificent regions there, which alone contain nearly 400 nebulæ and star-clusters 4." a The Heavens, Eng. ed., p. 395. The first who paid much attention to clusters and nebula was the French astronomer Messier, who formed the well-known and important though small Catalogue, the constituents of which are still distinguished by his initial-M. After him came Sir W. Herschel, who classified the nebulæ observed by him in the following way : I. "Bright nebulæ." II. "Faint nebula." III. "Very faint nebulæ." IV. “Planetary nebulæ, stars with bars, milky chevelures, short rays, remarkable shapes, &c." V. "Very large nebulæ." VI. "Very compressed rich clusters." VIII. "Coarsely scattered clusters." Objects catalogued by this observer are usually indicated by the symbol, with the number of the class in Roman capitals; thus-33 H VI Persei. References to Sir John Herschel's Catalogue of 1833, and his Cape extension of it, are indicated by the letter h with the number prefixed. For the great Catalogue of 1864 (the publication of which marks an era in this branch of the science) no designating letter has yet been agreed upon, but perhaps the capital H would be as convenient a one as could be chosen. The other observers who must be cited as having devoted much attention to nebula and clusters are the Earl of Rosse in England, and MM. D'Arrest and Schönfeld on the Continent. Lord Rosse laid before the Royal Society, in 1861, a large and valuable Catalogue of 989 nebulæ observed by himself at Parsonstown, which appeared in vol. cli. of the Philosophical Transactions. |