l62 I'HILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



hones, and the best too of the sort recommended in Smith's Optics; they should 

 be cemented in small pieces, in a kind of pavement agreeably to his directions, 

 on a thick round piece of marble, or metal made of lead and tin like the former 

 composition (which is what I have always used) in such a manner, that the lines 

 between the stones may run straight from one side to the other; so that, placino- 

 the teeth of a fine saw in each of these divisions, they may be cleared from one 

 end to the other of the cement which rises between the stones. This bed of 

 hones should be at least a 4th part larger than the metal to be ground on it. 

 The surface of the metal on which the hone pavement is to be cemented may or 

 may not, as you please, be turned of a convexity suitable to the gage, though I 

 have never taken that trouble. As soon as the hones are cemented down, and 

 the joints cleared by the saw, this tool must be fixed in the lathe, and turned 

 as exactly true to the gage as possible; which done, it must be laid aside for the 

 present. The next tool to be made is the bruiser. 



The manner of forming the bruiser, the fourth and last tool. — ^This tool 

 should be likewise made of thick stout brass, like the former, perfectly sound, 

 about a quarter of an inch thick, and hammered as near to the gage as possible. 

 It should be then scraped, cleaned, and tinned on the convex side, as the former 

 tool was on the concave, and the same thickness of lead and tin cast on it. 

 The general shape of this should differ from the former; for as that increased in 

 diameter at the bottom for the sake of standing firmly, so this should be only as 

 broad at bottom as at top, as it is to be used occasionally in both those positions. 

 When this tool is fixed in the lathe, and turned off concave to the convex gage 

 with great truth also, its diameter ought to be the middle size between the hones 

 and the polisher. Having with the lathe roughly formed the convex brass- 

 grinder, the bed of hones, and the concave bruisers, the convex and concave 

 brass tools and the metal must be wrought alternately and reciprocally on each 

 other with fine emery and water, so as to keep them as nearly to the same figure 

 as possible, in order to which some washed emery must be procured. This is 

 best done by putting it into a phial, which must be half filled with water and 

 well shaken up, so that, as it subsides, the coarsest may fall to the bottom first, 

 and the finest remain at the top: and whenever fresh emery is laid on the tools, 

 the best method (which we should also observe with the putty in polishing) will 

 be, to shake gently the bottle, and pour out a small quantity of the turbid 

 mixture. 



Of grinding the speculum, the brass tool, and the bruiser together. — All the 

 tools being ready, on a firm post in the middle of a room, begin to grind the 

 brass convex tool with the bruiser on it, working the latter crosswise, with 

 strokes sometimes across its diameter, at others a little to the right and left, and 

 always so short that the bruisers may not pass above half an inch within the surface 



