cf Metals for reflecting T'ekf copes. 341 
he fhewed me himfelf a box of finilhed metals, in which 
I am fure there were a dozen, and a half of the fame focal 
length. 
To return : a little ufe in working will make the whole 
of the procefs of grinding and polifhing very eafy and 
certain ; for though I have endeavoured to be as particu- 
lar as I can (I am almoft afraid too much fo) it is yet 
fcarcely poflible to fupply a want of dexterity, arifing 
from habit only, by the moll laboured and minute de- 
fcription. And though the above account may appear 
irkfome to the reader, as it lies cold before the eye, I am 
very fure, whoever attempts to make the inftrument, 
will not complain of it as tedioully particular. 
I will, however, farther remark, that when the metal 
begins to move ftiffly upon the polifher, and particularly 
when the figure is almoft brought to the parabolic form, 
it will be neceffary to fix the elbows againft the tides, in 
order to give momentum and equability to the motion 
of the hand by that of the whole body. 
The fame polifher will ferve for Jeveral metals, if it 
be fomewhat warmed when you begin to ufe it. 
There is another eircumftance, and a material one 
too, which muft not be omitted ; it is this. For the very 
fame reafon that the pitch lhould not be too hard or loft, 
the work will not proceed well in the heat of fummer, 
or 
