Ill 



THE NEBULAR HYPOTHESIS 



Acceptance of the Copernican system brought with 

 it the knowledge that the then known planets, Mercury, 

 Venus, the Earth, Mars, Jupiter, and Saturn, all re- 

 volve around the sun in the same direction. This 

 remarkable coincidence was rendered still more re- 

 markable by the discovery by Sir William Herschel, 

 in the year 1781, of the planet Uranus, which was short- 

 ly shown to be no exception to the general rule. About 

 the same time Herschel also taught the world that the 

 sky is dotted here and there with little clouds of lu- 

 minous matter, which he called nebulae (the word neb- 

 ula itself meaning "a little cloud"). At first he sup- 

 posed these to be clouds of gas, but as he successively 

 tried upon them telescopes of higher and higher power, 

 he found he could resolve nearly all of them into 

 clusters of stars. In the end it was only natural that 

 he should conclude all nebulae to be so resolvable, given 

 the requisite magnifying power; but in this it seems 

 he was mistaken, inasmuch as the spectroscopic evidence 

 shows some nebulae to be without visible nuclei of con- 

 densation. 



Swedenborg, who died before the discovery of 

 Uranus, and, following him, Kant, had sought to ex- 



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