of Metals for reflecting felef copes. 307 
nor will the fucceeding hone tool require fo much exadt- 
nefs, as any defe&s in turning, will, by a method hereafter 
mentioned, be eafily remedied; but any inequality or want 
of truth in the brafs tool will occafion a great deal of trou- 
ble before it can be ground out by the emery. This tool 
muft have a hole (fomewhat lefs than that in the metal 
to be worked upon it) in the middle, quite through to 
the bottom. When this tool is finiihed off in the lathe, 
its diameter fhould be one-eighth wider than the metal. 
Hozv to form the bed of hones , or the third tool. 
Having chofen the kind of hones, and the bell too, of 
the fort recommended in smith’s Optics; they fhould 
be cemented in fmall pieces (in a kind of pavement 
agreeably to his directions) upon a thick round piece of 
marble, or metal made of lead and tin like the former 
compofition (which is what I have always ufed) in fuch 
a manner, that the lines between the (tones may run 
ftraight from one fide to the other; fo that, placing the 
teeth of a fine faw in each of thefe divifions, they may 
be cleared from, one end to the other of the cement 
which rifes between the ftones. This bed of hones 
1 he be at leaft a fourth part larger than the metal 
which is to be ground upon it. The furface of the 
R r 2 metal 
