of Metals for reflecting 'Telefcopes . 319 
dation arifing from the. wear and yielding of both tool : 
and metal; whereas in this method, there is fuch a ftub- 
bornnels in the polifher, that the figure of the metal, 
good or bad, muft depend upon the truth of the former, , 
which is very feldom perfect. 
If the polifher be made in the fecond manner propo- 
fed, by flraining the pitch through an outer covering,, 
which is afterwards to be ftripped off, the fuperficies of 
pitch and farcenet is- fo very thin, that the putty, work- 
ing into them, forms a furface hard and untradlable, fo 
that it is impoffible to give the fpeculum a fine polifh. 
Accordingly all thofe metals which are wrought that way 
have an order of fcratches inftead of polifh, difcovering 
itfelf by a grey ifli vifible furface. Befides, fuppofing this 
tool perfectly finifhed, and anfwering its purpofe ever fb 
well, it is impoffible. it can produce in .the fpeculum any 
other than a fpherical .figure ; and indeed nothing elfe is 
expected from this method, as very evidently appears by 
the experiment recommended to afeertain the truth of 
the figure. You are directed to place a, final! luminous 
object in the center of the fphere of which the metal is 
a fegment, and then having adjufled an eye-glafs at the 
diftance of its own focal length from the object, and fo 
fituated that the image of the objedt formed by the fipe— 
culum may be. vifible to the eye, you are to judge of the 
3 perfect 
