of Metals for refleSling Telef copes. 315 
ftrokes round and round only, not carrying, however, the 
edges of the metal much over the hones ; this will take out 
the order of ftraight ftrokes : then having again wafhed 
the hones, and placed the fpeculum upon their center, 
with gentle preffure, Aide it towards you till its edge be 
brought a little over that of the hones, then carry it 
quite acrofs the diameter as far the other fide, and having 
given the metal a light ftrolce or two in this direction, 
take it off the tool. The metal being wiped quite dry, 
place it upon a table at a little di fiance from a window; 
ftand yourfelf as near the window, at fome diftance from 
the metal, and looking obliquely on its furface, turn it 
round its axis, and you will fee at every half turn the 
grain given by the laft crofs ftrokes flafh upon your eye 
at once over the whole face of the metal. This is as cer- 
tain a proof of a true fpherical figure as the operofe and 
difficult method defcribed in Dr. smith’s Optics; for as 
there is nothing foft or elaftic, either in the metal or in the 
hones, this glare is a certain proof of a perfect contact 
in every part of the two furfaces; which there could not 
be if the fpheres were not both perfect .and precifely the 
fame,. 
Indeed there is one accidental circumftance which ne- 
ceffarily affords its aid in this and every bufinefs of the 
like fort ; and that is, that a concave and convex furface 
S f 2 ground 
