THE CENTURY'S PROGRESS IN ASTRONOMY 



the photographic film reveal a prodigality of nebulous 

 matter in. the stellar system not hitherto even con- 

 jectured. 



And so, of course, all question of "island universes" 

 vanishes, and the nebulae are relegated to their true po- 

 sition as component parts of the one stellar system the 

 one universe that is open to present human inspection. 

 And these vast clouds of world-stuff have been found 

 by Professor Keeler, of the Lick Observatory, to be 

 floating through space at the starlike speed of from ten 

 to thirty-eight miles per second. 



The linking of nebula3 with stars, so clearly evi- 

 denced by all these modern observations, is, after all, 

 only the scientific corroboration of what the elder Her- 

 schel's later theories affirmed. But the nebulae have 

 other affinities not until recently suspected ; for the 

 spectra of some of them are practically identical with 

 the spectra of certain comets. The conclusion seems 

 warranted that comets are in point of fact minor nebu- 

 lae that are drawn into our system ; or, putting it other- 

 wise, that the telescopic nebulae are simply gigantic dis- 

 tant comets. 



Following up the suprising clews thus suggested, Mr. 

 J. Norman Lockyer, of London, has in recent years 

 elaborated what is perphaps the most comprehensive 

 cosmogonic guess that has ever been attempted. His 

 theory, known as the " meteoric hypothesis," probably 

 bears the same relation to the speculative thought of 

 our time that the nebular hypothesis of Laplace bore to 

 that of the eighteenth century. Outlined in a few 

 words, it is an attempt to explain all the major phe- 

 nomena of the universe as due, directly or indirectly, to 

 the gravitational impact of such meteoric particles, or 



