for Metals of reflecting Telefcopes . 317 
laft and moft effential procefs, viz, that of polifhing, I 
will defcribe it in the bell manner I can ; though many- 
little circumftances which will be unavoidably omitted 
(and which at the fame time are frequently effential to. 
the fuccefs of a mechanic procefs) can only be fupplied 
by adlual experience. 
The polilhing the fpeculum is the moft difficult and. 
effential part of the whole procefs; for every expe- 
rienced workman knows, to his vexation, that the moft 
trilling error here will be fufficient to fpoil the figure of 
his metal, and render all his preceding caution ufelefs. I 
have, however, difcovered a method which I lhall explain, 
not only of giving the metal a parabolic figure, but alfo of 
recovering it when it happens to be injured ; both to be 
effected in. the a£t of polifhing, and the former as cer- 
tainly as the fpherical figure is given upon the hones. 
Indeed, if we conlider rightly, polifhing will be perceived 
to be but a kind of grinding with a finer order of ftrokes, 
and with a powder infinitely finer than was before ufed 
in what is commonly called the grinding. But before I 
defcribe this method, which was the refult of many 
years experience, I will take the liberty of making fome 
few ftriddures on that of Meff. hadley -and moly- 
neux, which is followed by the generality of work- 
men. . 
