THE NEBULA IN ANDROMEDA. 77 



t 



review of its past history, he concludes that the periodical vari- 

 ations of the nebula extend to its figure, as well as to its bright- 

 ness. His grounds for this conclusion may be briefly stated as 

 follows : — 



1. The nebula is not found in any of the ancient catalogues. 



2. It was visible to the naked eye in the year 995, and its form 

 was then oval. 



3. For more than six hundred years afterwards it was unnoticed. 



4. The description given by Simon Marius of its appearance 

 in 1662 does not accord, in an important particular, — that of 

 exhibiting a central condensation, — with the observations of Le 

 Gentil in 1750. 



5. This condensation was not mentioned by Bouillaud in 1666, 

 who records an evident change of brilliancy between 1664 and 

 1666. 



6. Cassini, in 1740, represents its figure as triangular. 



7. Mairan, in 1754, regards the representation of Simon Marius 

 as essentially correct. 



8. His own observations indicated a round figure, of uniform 

 density throughout, in 1749 ; and an oval figure with a central con- 

 densation, in 1757-8. 



Although expressing himself convinced, by the foregoing con- 

 siderations, of the reality of a change, Le Gentil at the same time 

 suggests that these phenomena may be, in part, at least, explained 

 by referring them to the difference in the instrumental means em- 

 ployed by the several observers. His own telescopes were the 

 common refractors, of from three to thirty feet in focal distance, 

 in use before the invention of the achromatic object-glass, and 

 were of course very inferior to instruments of a more recent date. 



As all subsequent accounts of this nebula can, without violence. 



