304 Mr. mudge on the ConJlruEUon 
grinding and polifhing it. Four tools are all that are 
neceffary, viz. the rough grinder to work off the rough 
face of the metal ; a brafs convex grinder, on which the 
metal is to receive its fpherical figure ; a bed of hones 
which is to perfect that figure, and to give the metal a 
fine fmooth face; and a concave tool or bruifer, with 
which both the brafs grinder, and the hones are to be 
formed. A poliiher may be confidered as an addi- 
tional tool; but as the brafs grinder is ufed for this pur- 
pofe, and its pitchy furface is expeditioufly, and without 
difficulty formed by the bruifer, the apparatus is there- 
fore not enlarged. 
Of rough grinding the fpeculum. 
The tool by which the rough furface of the metal is 
rendered fmooth and fit for the hones, is belt made of lead, 
ftiffened with about a fifth or fixth part of tin. This tool 
Ihould be at leaft a third more in diameter than the metal 
which is to be ground ; and for one of any fize, not lefs 
than an inch thick. It may be cemented upon a block 
of wood, in order to raife it higher from the bench. 
This leaden tool being caft, it mult be fixed in the lathe, 
and turned as true as it is poffible, by the gage, to the 
figure of the intended fpeculum, making a hole or pit in 
the 
