274 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



To accomplish this, he substituted glass mirrors for metal ones, 

 and rendered them free from spherical aberration by working them 

 by a special method until they had obtained a nearly perfect 

 parabolic form. He also increased the reflecting power of the mirror 

 by silvering the surface. By using a solution of ammoniacal 

 nitrate of silver in alcohol, it is possible, at the ordinary temperature, 



FIG. 207. Leon Fouoault's telescope with silver mirror (Newtonian system). 



to cover the surface with a thick metallic film, which can be easily 

 renewed without at all damaging the geometric form of the mirror. 



An instrument of this kind, arranged after the Newtonian system 

 and mounted equatorially, so that the diurnal movement of a 

 heavenly body may be followed, lias recently been constructed at 



