326 Mr. A. E. Tutton. An Instrument for [Feb. 14, 



as far as the driving gear and the mode of supporting the grinding 

 table are concerned. The table itself is much larger however, the laps 

 being 4J ins. in diameter, and it is differently constructed, in order to 

 permit of rapid change from one lap to another. The table is pro- 

 vided with a permanent rigid gun-metal top 0, slightly larger than 

 the laps, upon which the latter are laid. The laps carry below three 

 cylindrical projections, which pass through corresponding holes in 

 the table, and they may be locked in this position by a plate carried 

 beneath the table, and which is rotatable for a sufficient amount by a 

 rack and pinion actuated by a short lever ; the plate is pierced by 

 tapering slots, wide enough at one end to allow the projections to fall 

 through or be removed, but narrowed at the other so as to gear in 

 niches cut in the projections. The lap h is placed in position, or 

 lifted out when it is desired, to remove it, while the lever is rotated 

 outwards radially to the axis of the table ; upon rotating the lever 

 inwards again, as far as a stop permits it to go under the table, the 

 lap, if in position, will be firmly locked to the latter. 



Four metallic laps are provided ; one of iron, for rough grinding 

 with coarse emery and brick oil or water ; two of gun-metal and 

 hard white metal respectively, for fine grinding with flour emery ; 

 and another of pewter, for polishing with rottenstone and water. A 

 polishing lap of hard felt, for use with putty powder and water, and 

 a lap of boxwood, either for grinding soft mineral crystals or for 

 polishing, are likewise supplied. There are also three glass laps for 

 use with artificial crystals, one moderately ground for rough grinding, 

 the second extremely finely ground, and another of plain plate glass, 

 the last two for polishing, and all three to be lubricated with oil. 

 The final polisher of ordinary plate glass furnishes admirably 

 polished surfaces, and best of all when the amount of oil is extremely 

 small. 



The cutting apparatus is carried at a suitable height above the 

 level of the grinding table upon a rigid horizontal arm pivoted upon 

 the back pillar k of the instrument, so that it can be completely 

 rotated out of the way during grinding operations, and further sup- 

 ported when required for use upon an adjunct of the right front 

 pillar I. It consists of a 4-in. disc of soft iron m, supplied with 

 diamond edge, and intended to be lubricated with brick oil, supported 

 truly parallel to the grinding table between two stout broad washers, 

 carried by the almost frictionless axle, and driven by an independent 

 driving gear ; the latter is somewhat similarly arranged to that of 

 the grinding apparatus, in order that the pressure of the band round 

 the pulley which turns with the axle of the cutter shall be equal on 

 each side to minimise friction, and is entirely carried upon the arm. 

 The latter is bent inwards towards the crystal, so that the cutting 

 edge may be conveniently approached. 



