of Metals for refle&ing ft def copes. 333 
aberration ; indeed the deviation will be fo great, as to be 
very fenfibly perceived by a great indiftindtnefs in the 
image. 
How to give the parabolic figure to the metal. 
In order then to give the fpeculum the laft and finilh- 
ing figure, which is done by a few ftrokes, it muft be 
particularly remarked, that by working the metal round 
and round, the fphere of the polilher by this means 
growing lets, it wears fafteft in the middle : and as a 
legment of a fphere may become parabolic, by open- 
ing the extremes gradually from within outwards, fo 
it may be equally well done by increafing the curva- 
ture in the middle, in a certain ratio,, from without kw 
wards. 
Suppofing then the metal to be now truly fpherical, 
flop the hole in the polilher, by forcing a cork into it 
underneath, about an inch, fo that it do not reach quite 
to the furface; and having w allied off any mud that may 
be on the furface of the took with a wet foft piece of 
fponge, whilft the furface of it is a little moift, place the 
center of the metal upon the middle of the polilher; 
then having, with the wet brulh, lodged as much water 
round the edge of the metal as the projecting edge will 
. hold. 
