222 



heavenly body from the place in which it had before been 

 distinctly seen, may be the result of its own motion as much 

 as of any such diminution of its photometric process (whether 

 on its surface or in its photosphere), as would render the 

 waves of light too weak to excite our organs of sight. 

 What we no longer see, is not necessarily annihilated. The 

 idea of destruction or combustion, as applied to disappearing 

 stars, belongs to the age of Tycho Brahe. Even Pliny, 

 in the fine passage where he is speaking of Hippar- 

 chus, makes it a question: Stellas an obirent nasceren- 

 turve ? The apparent eternal cosmical alternation of existence 

 and destruction is not annihilation ; it is merely the transition 

 of matter into new forms, into combinations which are sub- 

 ject to new processes. Dark cosmical bodies may by a 

 renewed process of light again become luminous. 



PERIODICALLY TARIABLE STARS. Since all is in 

 motion in the vault of heaven, and everything is variable 

 both in space and time, we are led by analogy to infer 

 that as the fixed stars universally have not merely an appa- 

 rent, but also a proper motion of their own, so their surfaces 

 or luminous atmospheres are generally subject to those 

 changes which recur, in the great majority, in extremely long 



tus also ascribes to Cygnus none but stars " of moderate 

 brilliancy," Hipparchus expressly refutes this error, and 

 adds the remark, that the bright star in the Swan (Deneb) 

 is little inferior in brilliancy to Lyra (Vega Lyree). Pto- 

 lemy classes Vega among stars of the 1st magnitude, and 

 in the Catasterisms of Eratosthenes ( cap. 25 ), Vega is 

 called \CVKOV KOI \ap.Trpbv. Considering the many inaccuracies 

 of a poet, who never himself observed the stars, one is 

 not much disposed to give credit to the assertion that it was 

 only between the years 272 and 127 B.C., i.e., between 

 the times of Aratus and Hipparchus, that the star Vega 

 Lyrse (Fidicula of Pliny, xviii. 25,) became a star of the 1st 

 magnitude 



