vi The Solar System and Universe 95 



tions of astronomers have laboured to discover the nature 

 of these cloudy tracts, and in many cases they have succeeded 

 in showing them to be clusters of immeasurably remote stars v 

 The forms of these star -clusters or remote universes are 

 in many cases wonderfully beautiful ring-shaped, oval, 

 rod -like, or resembling dumb-bells or spirals of much 

 complexity. 



142. Nebulae. The old observers were accustomed to 

 find that many nebulae which their telescopes only showed 

 as a gauzy cloud were resolved into star-clusters when a 

 more powerful instrument was brought to bear on them. 

 Consequently it was long believed that all unresolved nebulas 

 were simply star-clusters that larger telescopes could make 

 plain. When Mr. Huggins first succeeded in observing 

 the spectra of the unresolved nebulas in 1864 he detected 

 bright lines unlike those of stars, and doubtless coming from 

 intensely heated gases. The nebulas were therefore sup- 

 posed to be distant masses of glowing gas. Professor Nor- 

 man Lockyer has recently suggested a somewhat different 

 explanation of the spectrum. He points out that the spectra 

 of nebulae and of comets' tails and of meteorites in a vacuum 

 tube ( 133, 135) are all so much alike that they are prob- 

 ably produced by the same materials. Following an earlier 

 suggestion of Professor P. G. Tait, he views a nebula as a 

 vast swarm of meteorites moving in different directions, and 

 by dashing against each other producing heat enough to 

 drive a part of their substance into luminous vapour. 



143. The Nebular Hypothesis. The Prussian philo- 

 sopher Kant and subsequently the French astronomer 

 Laplace accounted for the origin of the solar system by sup- 

 posing that at one time in the remote past it consisted 

 merely of a vast nebula or cloud of intensely hot gas 

 extending far beyond the orbit of the outermost planet. 

 As this cloud cooled and contracted it acquired a whirling 

 motion from west to east, and formed a rotating gaseous 

 disc which gradually condensed at the centre to form the 

 embryo Sun. The edge of the whirling disc was thrown 

 off as a ring by centrifugal force, and the ring ultimately 

 condensed into the planet Neptune. The gaseous disc 



