telescope. In 1868, O. Struve, observing at Pulkova, detected another nebulous spot in the vicinity of the place of the missing object, but this has also now vanished. Struve does not, however, consider that the nebula of 1868 is distinct from Hind's nebula, but he says:
This is the best authenticated history of observed change in any nebula. 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 discoverer was confident that no corresponding object existed on the evening of the 20th of November. The brilliancy of the new star gradually declined, until, on the 13th of December, Mr. Hind found it of the sixth magnitude. The spectrum of this star was carefully studied by many observers, and it exhibited several bright lines, which indicated that the star differed from other stars by the possession of vast masses of glowing gaseous material. This star was observed by Dr. Copeland, at the Earl of Crawford's observatory, on September 2, 1877. It was then below the tenth magnitude, and of a decidedly bluish tint. Viewed through the spectroscope, the light of this star was almost completely monochromatic, and appeared to be indistinguishable from that which is often found to come from nebulæ. Dr. Copeland thus concludes:
It should, however, be added that Professor Pickering has since found slight traces of a continuous spectrum, but the object has now become so extremely faint that such observations are very difficult. This remarkable history might be adduced if we wished to procure evidence of the conversion of stars into nebulæ, but for the nebular theory we require evidence of the conversion of nebulæ into stars.
Care must be taken not to exaggerate the inferences to be drawn from the two instances I have quoted, viz., the variable nebula in Taurus and the new star in Cygnus. I think it more likely that both of