_Variable Nebulæ._—It is in the highest degree probable that changes occur in the physical appearances of certain nebulæ, though the opinion is not perhaps supported by a sufficient number of instances. Until Sir W. Herschel began his review of the heavens very few nebulæ were known, and the information possessed about them was very incomplete. The early records, obtained with small and inferior telescopes, scarcely admit of comparison with recent observations, for in matters of detail little agreement will be found; and this proceeds certainly not so much from real changes in the objects as from differences due to the variety of instruments employed, to atmospheric vagaries, and to “personal equation.” Bullialdus and Kirch in 1667 and 1676 and Le Gentil in 1759 supposed that remarkable changes were operating in the great elliptical nebula of Andromeda. But G. P. Bond fully investigated the evidence, and concluded that the variability of the object was by no means proved. Some observers have represented the nucleus as stellar, while others have drawn it as a gradual condensation, and Dr. Copeland has shown that different magnifying powers alter the aspect of the nucleus, “the lower powers making it more star-like, the higher ones more soft-looking and extensive.” Mairan and others entertained the view that the large irregular nebula in Orion was subject to change. This object received much attention from Sir W. Herschel, and he concluded that it underwent great alteration between 1774 and 1811. D’Arrest, from his own researches and a discussion of other results, expressed himself in 1872 that “the observed changes in this vast mass of gas seem exclusively to turn out to be temporary fluctuations of brightness.” Prof. Holden has arrived at a similar conclusion, and says:—“The figure of the nebula has remained the same from 1758 till now (if we except a change in its apex about 1770, which seems quite possible); but in the brightness of its parts undoubted variations have taken place, and such changes are still going on”[59] (‘Monograph of the Nebula in Orion,’ p. 225). Hind discovered a faint nebula, with a diameter of about 1′, on Oct. 11, 1852. It was situated in Taurus, the position being R.A. 4^h 15^m 33^s, Dec. +19° 15′·6 (for 1890), or about 2° W. of the star ε Tauri (mag. 3·7). D’Arrest, on Oct. 3, 1861, searched for this object, but found it had quite disappeared! A small round nebula was seen in 1868, about 4′ preceding Hind’s, but this resisted some later attempts at observation. In Oct. 1890, Burnham and Barnard, with the 36-inch refractor of the Lick Observatory, saw two nebulæ here, one a very small, condensed nebula, with a stellar nucleus, and the other an exceedingly faint nebulosity about 45″ in diameter (see ‘Monthly Notices,’ vol. li. pp. 94, 95).