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



surface is exactly parallel to the cutting disc and grinding lap, is 

 carried out as described in the former communication. When about 

 to commence cutting, the crystal is first lowered by means of the 

 milled head at the top of the instrument until it is at a convenient 

 level for cutting, in which position it is fixed by first clamping the 

 circle to the fixed cone by means of the milled headed screw forming 

 part of the arrangement for fine adjustment, and then preventing 

 any movement of the gun-metal axis by tightening the collar imme- 

 diately above the circle by means of a key provided. Cutting is 

 then proceeded with, commencing rotation of the cutter and its tra- 

 versing slowly at first, and gradually increasing the speed according 

 to the " feel" of the catting. If by inadvertence a jam should ever 

 occur, a slight reversal of the motion of the traversing screw will 

 instantly release the cutter. The speed should be materially reduced 

 towards the finish, in order that the end of the crystal may be cut off 

 cleanly right up to the furthest edge. A hard crystal such as a 

 topaz, ^ in. thick, may be easily cut through in 10 to 15 minutes, 

 and the cut surface is so smooth that very little grinding is required, 

 which may at once be proceeded with upon the gun-metal or hard 

 white-metal lap, with the finest washed flour emery made into a 

 moderately thick paste with water or brick-oil. The latter lubricant 

 offers the advantage of greater immunity from splashing, although 

 not perhaps so convenient in other respects. 



A thick glass disc, about 1 in. in diameter, is provided for use as a 

 " bruiser," to work down the emery paste to an even consistency upon 

 the lap before grinding. The pressure between crystal and lap is 

 controlled by manipulation of the counterpoised levers, as described 

 in the former memoir, and the path of the crystal upon the lap 

 changed from time to time by use of the centering movements, in 

 order to prevent undue local wearing of the lap. 



A further useful accessory is a small stand consisting of a weighted 

 base carrying an upright tubular stem, in which a lengthening rod 

 is capable of sliding, which latter carries an adjustable support, 

 formed by a couple of short links united by ball-and-socket joints, for 

 an oblique caoutchouc wedge. The latter, when directed down upon 

 the lap, not quite in contact with the metallic surface of the latter, 

 serves to keep the emery paste moving into the path of the crystal 

 during grinding. When soft artificial crystals are being ground 

 upon the glass laps, the wedge may be replaced by a camel-hair 

 brush, to keep the path well supplied with the lubricating oil. 



The surfaces prepared by the fine grinding are usually sufficiently 

 transparent for the purposes of optical investigations, plates perpen- 

 dicular to the median lines exhibiting clearly defined interference 

 figures in convergent polarised light. If a higher polish is desired, 

 the polishing laps supplied will enable this to be attained. 



