Page:The American Cyclopædia (1879) Volume XII.djvu/208

 200 NEBULA simple globes of nebulous matter, as was for- merly supposed. There are several which have perfectly the appearance of a ring, and are called annular nebulae. A conspicuous and beautiful example is situated in Lyra. Some appear to be physically connected in pairs like double stars. Most of the small nebulas have the general appearance of a bright central | nucleus enveloped in a nebulous veil. This nu- j cleus is sometimes concentrated as a star and sometimes diffused. The enveloping veil is sometimes circular and sometimes elliptical, with every degree of eccentricity between a cir- cle and a straight line. There are some which, with a general disposition to symmetry of form, ! have great branching arms or filaments with | more or less precision of outline. An example of this is Lord Rosse's Crab nebula. Another j remarkable object is the nebula in Andromeda, ! which is visible with the naked eye, and is the only one which was discovered before the invention of the telescope. Simon Marius (1612) describes its appearance as that of a candle shining through horn. Besides the above, which have comparatively regular forms, there are others more diffused, and devoid of symmetry of shape. A remarkable ex- ample is the great nebula in Orion, discov- ered by Huygens in 1656. This nebula and that in Andromeda have been admirably de- lineated by the professors Bond of Harvard observatory. (See "Memoirs of the Amer- ican Academy of Arts and Sciences," new series, vol. iii.) The great nebula in Argo, which Sir John Herschel has charted with exquisite care and elaborateness in his "Cape Observations," is another example of this class. In the southern firmament there are two ex- tensive nebulous tracts known as the Ma- wonderful objects which have been recognized in all the heavens is upward of 5,000. Of these fewer than 150 were known prior to the time of Sir William Herschel. In 1786 he communicated to the royal society a catalogue of 1,000 new nebulae and clusters; in 1789 a FIG. 5. Great Nebula in Orion. gellanic clouds; the greater called Nubecula Major, and occupying an area of 42 square degrees; the smaller called Nubecula Minor, and covering about 10 square degrees. In these tracts are found -multitudes of small nebulas and clusters. The number of these FIG. 6. Large Magellanic Cloud. second catalogue of the same number of new objects; and in 1802 a third which included 500 more. In 1833 Sir John Herschel com- municated to the royal society a catalogue of 2,306 nebulae and clusters in the northern hemi- sphere observed by him, 500 of which were new. In 1847 appeared his " Cape Observa- tions," which contained catalogues of 1,708 nebulas and clusters in the southern heavens. The application of spectroscopic analysis to these objects, by Huggins, Secchi, Vogel, and others, has resulted in the noteworthy discov- ery that while some among the nebulas are really clusters of stars, others consist in the main of gaseous matter. The former give spectra resembling in their general character- istics the spectra of stars; the latter give a spectrum of three bright lines (occasionally four), one line corresponding in position to a line in the spectrum of hydrogen, another cor- responding to a line in the spectrum of nitro- gen. The resolvable nebulas mostly give spec- tra of the former class, while the bright-line spectrum is given by all the irregular nebulre hitherto examined, and by the planetary nebulas. Of about 70 nebulas examined by Huggins, nearly one third gave the spectrum indicative of gaseity, the rest giving a stellar spectrum. As to the nature of nebulas, two chief theories have been advanced. It was first suggested by Wright of Durham, and afterward maintained by Kant and Lambert, that the nebulas are stellar galaxies similar to our own star system. Sir W. Herschel, at the beginning of his re- searches into the constitution of the universe, adopted this view as respects certain nebulas which he regarded as external, while holding