214 Mary Somerville. 



optical power. In 1782 Sir William Herschel saw 

 one of the stars of Zeta Herculis eclipse the 

 other. 



In the " Connexion of the Physical Sciences " I 

 have given an' abridged account of Sir John 

 HerscheFs most remarkable discoveries in the 

 southern hemisphere ; but I may mention here that 

 he determined the position and made accurate draw- 

 ings of all the nebulae that were distinctly visible in 

 his 20 ft. telescope. The work he published will be 

 a standard for ascertaining the changes that may 

 take place in these mysterious objects for ages to 

 come. Sir William Herschel had determined the 

 places of 2,500 nebulae in the northern hemisphere ; 

 they were examined by his son, and drawings made 

 of some of the most remarkable, but when these 

 nebulae were viewed through Lord Rosse's telescope, 

 they presented a very different appearance, showing 

 that the apparent form of the nebulae depends upon 

 the space- penetrating power of the telescope, a cir- 

 cumstance of vital importance in observing the 

 changes which time may produce on these wonder- 

 ful objects. 



[Long afterwards Lord Eosse wrote in reply to some 

 questions which my mother had addressed to him on 

 this subject : 



