THE SIDEREAL UNIVERSE. 739 



the nebulae. Sir John Herschel says they are formed of 

 isolated stars, of swarms of stars, and lastly, of nebula?, 

 more compact than those which we find near the Virgin 

 and in the Tresses of Berenice. 



The first mariners who ventured into the southern 

 seas were also struck by certain phenomena of a totally 

 opposite character; these were black patches irregularly 

 outlined on the vault of the heavens, to which in their 

 imaginative language they gave the name of coal-sacks. 

 According to astronomers, these patches, the most cele- 

 brated of which are near the Southern Cross, are due to 

 the sky being at these parts to a great extent without 

 stars. They seem to be really holes, according to the 

 expression of Humboldt, by means of Avhich our vision 

 pierces into the remotest spaces of the universe. 



