HERSCHEL THE DISCOVERER. 35 



he found that the nebulae were aggregated near 

 the galactic poles ; in other words, where nebulae 

 are numerous, stars are scarce, and vice versa. 

 So rigorously did this rule hold, that when 

 dictating his observations to his sister Caroline, 

 he would, on noting a paucity of stars, warn her 

 to " prepare for nebulae." 



" A knowledge of the construction of the 

 heavens has always been the ultimate object of 

 my observations." So Herschel wrote in 1811. 

 All his investigations were secondary to the 

 problem which was constantly before his mind 

 the extent and structure of the Universe. 

 He aspired to be the Copernicus of the Sidereal 

 System. Although Bruno, Kepler, Wright, Kant, 

 and Lambert had speculated regarding the con- 

 struction of the heavens, they had not the 

 slightest evidence on which to base their ideas. 

 There was no science of sidereal astronomy. The 

 stars were observed only to assist navigation, 

 and the primary object of star-catalogues was to 

 further knowledge of the motions of the planets. 

 In Herschel's day, also, the distances of the stars 

 had not been measured, and he had to base his 

 views on the distribution of the stars. In 1784, 

 therefore, he commenced a survey of the heavens, 

 in order to ascertain the number of stars in 

 various parts of the sky. This method, which 



