THE BOUNDARIES OF ASTRONOMY. 241 



telescope. In 1868, 0. 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 18G8 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. 



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 indistin- 

 guishable from that which is often found to come from nebulse. Dr. 

 Copeland thus concludes : . 



Bearing in mind the history of this star from the time of its discovery by 

 Schmidt, 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. At least it 

 may be safely affirmed that no astronomer, discovering the object in its present 

 state, would, after viewing it through a prism, hesitate to pronounce as to its 

 present nebulous character. 



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 re- 

 markable history might be adduced if we wished to procure evidence 

 of the conversion of stars into nebula?, but for the nebular theory we 

 require evidence of the conversion of nebula? 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 



VOL. XXIII. 16 



