In the recent work of Professor Ball, "In the High Heavens," that author says, "There are, however, good grounds for believing that nebulæ really do undergo some changes, at least as regards brightness; but whether these changes are such as Herschel's theory would seem to require is quite another question. Perhaps the best-authenticated instance is that of the variable nebula in the constellation of Taurus, discovered by Mr. Hind in 1852. At the time of its discovery this object was a small nebula about one minute in diameter, with a central condensation of light. D'Arrest, the distinguished astronomer of Copenhagen, found in 1861 that this nebula had vanished. On the 29th of December, 1861, the nebula was again seen in the powerful refractor at Pulkova, but on December 12, 1863, Mr. Hind failed to detect it with the telescope by which it had been originally discovered.... In 1868, O. Struve, observing at Pulkova, detected another nebulous spot in the vicinity of the place of the missing object, but this also has now vanished. Struve, however, does not consider that the nebula of 1868 is distinct from Hind's nebula, but he says, 'What I see is certainly the variable nebula itself, only in altered brightness and spread over a larger space. Some traces of nebulosity are still to be seen exactly on the spot where Hind and D'Arrest placed the variable nebula. It is a remarkable circumstance that this nebula is in the vicinity of a variable star which changes somewhat irregularly from the ninth to the twelfth magnitude. At the time of the discovery in 1861 both the star and the nebula were brighter than they have since become.'... It must be admitted that the changes are such as would not be expected if Herschel's theory were universally true. Another remarkable occurrence in modern astronomy may be cited as having some bearing on the question as to the actual evidence for or against Herschel's theory. On November 24, 1876, Dr. Schmidt noticed a new star of the third magnitude in the constellation Cygnus.... The brilliancy gradually declined until, on the 13th of December, Mr. Hind found it to be of the sixth magnitude. The spectrum ... exhibited several bright lines which indicated that the star differed from other stars by the possession of vast masses of glowing gaseous material.... September 2, 1877, it was then below the tenth magnitude and of a decidedly bluish tint. Viewed through the spectroscope, its light was almost completely monochromatic, and appeared to be indistinguishable from that which is often found to come from nebulæ.... It would seem certain that we have an instance before us in which a star has changed into a planetary nebula of small angular diameter.... Professor Pickering, however, has since found slight traces of a continuous spectrum, but the object has now become so extremely faint that such observations are very difficult.... For the nebular theory we require evidence