AIDS OF THE NEWTONIAN PERIOD. 281 



peculiar effect, till the elder Herschel made them 

 his especial study. His skill and perseverance in 

 grinding specula, and in contriving the best ap- 

 paratus for their use, were rewarded by a number 

 of curious and striking discoveries, among which, as 

 we have already related, was the discovery of a 

 new planet beyond Saturn. In 1789, Herschel sur- 

 passed all his former attempts, by bringing into 

 action a reflecting telescope of forty feet length, 

 with a speculum of four feet in diameter. The first 

 application of this magnificent instrument showed a 

 new satellite (the sixth) of Saturn. He and his son 

 have, with reflectors of twenty feet, made a com- 

 plete survey of the heavens, so far as they are 

 visible in this country; and the latter is now in a 

 distant region completing this survey, by adding to 

 it the other hemisphere. 



In speaking of the improvements of telescopes 

 we ought to notice, that they have been pursued in 

 the eye-glasses as well as in the object-glasses. In- 

 stead of the single lens, Huyghens substituted an 

 eye-piece of two lenses, which, though introduced 

 for another purpose, attained the object of destroy- 

 ing colour 6 . Ramsden's eye-piece is one fit to be 

 used with a micrometer, and others of more com- 

 plex construction have been used for various pur- 

 poses. 



6 Coddington's Optics, ii. 21. 



