COMETS. 135 



identified in its spectrum the prominent iron-lines 

 as well as the sodium-line. These observations 

 were certainly confirmatory of Bredikhine's theory. 

 It should also be stated, however, that several 

 comets have shown, in addition to the hydro- 

 carbon spectrum, that of reflected sunlight, which 

 proves that the light we receive from comets 

 is of a compound nature. 



The comet which appeared in 1880 was an- 

 nounced by Benjamin Apthorp Gould (1824-1896) 

 to be a return of the great comet of 1843. Cal- 

 culations by Gould, Copeland, and Hind revealed 

 a close similarity between the elements of the 

 two orbits. Eventually it had to be admitted 

 that the comets were separate bodies travelling 

 in the same orbit. Then, two years later, the 

 great September comet of 1882 was found to 

 revolve in the same orbit as those of 1668, 1843, 

 and 1880. Four years later, another comet, dis- 

 covered in 1887, was found to move in the same 

 path. 



Closely allied to this subject is the existence 

 of " comet families," demonstrated by Hoek of 

 Utrecht in 1865, and mentioned in our chapter 

 on the Outer Planets. These comets are found 

 to be dependent on the planets, Jupiter, Saturn, 

 Uranus, and Neptune, each possessing a comet- 

 group. Various theories have been advanced to 



