NEBULAE

These are of especial interest, for the reason that they have played so large a part in forming cosmic theories—the Laplacian, the Planetestimal, etc. Nebulæ are of various kinds—“Annular Nebulæ,” resembling a flat, oval, solid ring, having a dark hole in the center. Then there are “Elliptic Nebulæ,” of varying degrees of eccentricity; the Great Nebula in Andromeda being a good example. (Numbers of isolated stars may be found within its limits.) “Spiral Nebulæ” are, perhaps, the best known of all, and their name accurately describes their appearance. There are also the so-called “Planetary Nebulæ,” as well as Nebulous Stars, Irregular Nebulæ, etc. Of late years, much interest has been centered upon the so-called “Dark Nebulæ.” Herschel had long before described various “holes in the heavens,” wherein no stars could be discerned. It is now believed that such spots do not represent “holes,” as much as dark masses of matter, which seem to blot out the bright stars behind them. The interested reader may refer to Hale’s “The Depths of the Universe” for additional information upon this topic, which is relatively new to astronomy. It is also interesting to note that the spectra of Nebulæ contain the bright lines in the green of a substance called “nebulium,” because it is not found except in nebulæ.