C 3 
thought an Attempt to render it more fo, would not 
be altogether ufelefs, or yet unworthy the Notice of 
the Curious. 
VIII* A new Method of mpron^mg and per» 
fedlng Catadioptrical Telefcopes, hy form-> 
ing the Speculums of Glafs injtead of Metal, 
By Caleb Smith. 
T he Telefcope is defervedly reckoned one of 
the moft excellent of all the Inventions of the 
Moderns j fuch noble and ufeful Difcoveries have 
been made by means of this admirable Inftrument, 
and are hill to be expected from its further Improve- 
ment, that many of the moft eminent Mathemati- 
cians have imployed their utmoft Skill and Induftry 
to bring it to Perfedion. 
The Imperfedlions of Telefcopes are attributed to 
two Caufes; to wit, The Unfitnefs of the Spherical 
Figure to which the Glaffes are ufually ground, and 
the different Refrangibility of the Rays of Light. 
The firft of thefe Defeds only, was known to the 
Writers of Dioptries, before Sir Ifaac Newt on '■> for 
which Reafon (as he informs us himfelf. Opt. Le6i. 
I, 2 .) they imagined, that Optical Inftruments 
** might be brought to any Degree of Perfeftion, 
“ provided they were able to communicate to the 
Glaffes, in grinding, what Geometrical Figure they 
pleafed; to which Purpofe various Mechanical 
Contrivances, were thought of, whereby Glaffes 
y might be ground into Hyperbolical, or even Para- 
bolical. 
