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streak behind the nucleus (which, however, is in this case somewhat eccentrically situated). This phenomenon is not easily explained. It is common, indeed, to find it accounted for as due to the absence of illuminated matter behind the nucleus. But as a comet s tail is not a flat object, spread out on the dome of the heavens, but a conical, or rather conoidal, appendage, which viewed from all sides would present, no doubt, much the same appearance, the line of sight directed towards the axis must pass through the illuminated en velope. Huggins s spectroscopic examination of this comet yielded results resembling those he had already obtained from small comets. But this was the first comet whose tail he had analysed. The spectroscopic analysis of the

various parts of the comet gave the following results:—

Fig. 51.—Coggia's Comet.

" When the slit of the spectroscope was placed across tlie nucleus and coma, there was seen in the instrument a broad spectrum, con sisting of the same three bright bands exhibited by Comet II., 1868, crossed by a linear continuous spectrum from the light of the nucleus. On the continuous spectrum of the nucleus," he proceeds, " I was not able to distinguish with certainty any dark lines of absorption, or any bright lines, other than the three bright bands. Besides these spectra, there was also present a faint broad continu ous spectrum between and beyond the bright bands. When the slit was moved on to dilferent parts of the coma, the bright bands and the faint continuous spectrum were found to vary in relative intensity. When the slit was brought past the nucleus on to the commencement of the tail, the gaseous spectrum became rapidly fainter, until, at a short distance from the nucleus, the continuous spectrum predominated so strongly that the middle band only, which is the brightest, could be detected on it. We have presented to us, therefore, by the light of the comet, three spectra : (1.) The spectrum of bright bands; (2.) The continuous spectrum of the nucleus; (3.) The continuous spectrum which accompanies the gaseous spectrum in the coma, and which represents almost entirely the light of the tail."

Chapter XVIII.—The Fixed Stars and Nebulæ.

The great multitude of stars visible even to the naked eye renders it impossible to distinguish each by a particular name ; astronomers, accordingly, for the sake of re ference, have formed them into groups, known as Constellations or Asterisms. To these the early astronomers gave the names of men and animals, and other objects, from fancied resemblances or analogies, for the most part not easily traced. These denominations are preserved in modern catalogues ; and the practice of delineating the object itself on celestial globes and charts has been only recently, if it has yet been altogether, abandoned. The ancients distinguished some of the brightest stars in the different constellations by particular names ; but to include others less conspicuous, it became necessary to adopt a different mode of proceeding. According to the usual method, first introduced by Bayer in his Uranometria, each of the stars in every constellation is marked by a letter of the Greek alphabet, commencing with the most brilliant, which is designated by a; the next most con spicuous is called /?, the third y, and so on. When the Greek letters are exhausted, recourse is had to the Roman or Italic. The order of the letters was intended to indicate only the relative brilliancy of the stars in the same constellation, without any reference to those in other parts of the heavens. Thus a Aquarii is a star of the same order of brightness as y Virginia. Moreover, whether owing to carelessness in the original lettering or to changes in the brightness of the stars, the order of the letters does not always correspond to the brightness, even in the same constellation. Thus, e, f, and 77 Ursse Majoris are far brighter than 8 of that constellation. The stars of the first order of brightness are likewise denominated stars of the first magnitude ; those of a degree inferior in brightness are said to be of the second magnitude, and so on with the third, fourth, &c. Below the sixth the same denomina tions are continued ; but the stars of the seventh and inferior magnitudes are no longer visible to the naked eye. and are therefore called telescopic stars.

The number of constellations given by Ptolemy is 48, viz.:—

North of the Zodiac.

Ursa Minor, Ursa Major, Draco, Cepheus, Bootes, or Arctophylax-, Corona Borealis, Hercules ^Ev y6va.ffu>), Lyra, Cygnus, Cassiopeia, Perseus, Auriga, Serpentarius, or Ophiuchus, Serpens, Sagitta, Delphinus, Equuleus, Pegasus, or Equus, Andromeda, Triangulum, Aries, Taurus, Gemini, Cancer, The Little Bear. The Great Bear. The Dragon. Cepheus. The Herdsman, or the Bearwarden. The Northern Crown. Hercules (kneeling). The Harp. The Swan. The Lady in her Chair. Perseus. The Waggoner. The Serpent-bearer. The Serpent. The Arrow. ,The Eagle and Antinous. The Dolphin. The Horse s Head. The Flying Horse, or the Horaa, Andromeda. The Triangle.

In the Zodiac.

26. Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn us, Aquarixis, Pisces, The Ham. The Bull. The Twins. The Crab. ( The Lion, to which Ptolemy joined ( some stars of Berenice s hair. The Virgin. The Balance. The Scorpion. The Archer. The Goat. The Water-Bcarer. The Fishes.

Southern Constellations.

34. Cetus, 35. Orion, 36. Eridanus, or Fluvius, 37. Lepus, 38. Canis Major, 39. Canis Minor, 40. Argo Navis, 41. Hydra, 42. Crater, 43. Corvus, 44. Centaurus, 45. Lupus, 46. Ara, 47. Corona Australis, 48. Piscis Australis. The Whale. Orion. Eridanus, or the Kiv The Hare. The Great Dog. The Little Dog. The Ship Argo. The Hydra. The Cup. The Crow. The Centaur. The Wolf. The Altar. The Southern Cri>ivn.

The Southern Fish.