1895.] Cutting, Grinding, and Polishing Section-plates, fyc. 329 



After the completion of the operations upon the first surface, the 

 cutting of the second s is proceeded with while the crystal is still 

 adjusted, the latter being fixed at the suitable height for the purpose. 

 The plate thus cut off is cemented by the finished surface to one of 

 the small glass discs provided with the special holder, and the 

 finishing of the second surface carried out with the disc supported 

 in the holder, whose true plane has previously been adjusted parallel 

 to the lap, as described in the former communication. 



When a prism is to be prepared the crystal is first adjusted so that 

 the plane of optical elasticity to which the two desired prism faces 

 are to be symmetrical is parallel to the grinding lap, and likewise 

 so that the direction of the desired edge, parallel to an axis of optical 

 elasticity, is at right angles to the upper tangent screw of the adjust- 

 ing apparatus. The latter is then rotated 30, the crystal arranged 

 at the convenient height for cutting, and the end cut off. The cut 

 surface is then finished upon the laps. The tangent screw is next 

 rotated back to the adjusted position, and for 30 upon the other side 

 of that, when, after fixing at the proper height, the second surface is 

 cut. The 60 prism thus cut off is again set in wax on the holder 

 with the second surface exposed, and the latter is then brought 

 parallel to the lap by goniometrical adjustment, and ground and 

 polished, if desired, in like manner to the first. The two surfaces 

 will invariably afford brilliant single reflected images of the spectro- 

 meter slit, and transmit equally well-defined refracted images when 

 arranged for minimum deviation. 



Instead of actuating the driving gear of the cutting or grinding 

 apparatus by hand, which, however, is very light labour owing to the 

 freedom from friction, a small electric, gas, or water motor may be 

 employed. For this purpose the driving wheel a, of the grinding 

 apparatus is supplied with two annular niches, the upper of which is 

 intended for the reception of the band from the motor ; and two 

 similar driving pulleys, one, /3, for hand rotation, and the other, 7, for 

 use with the motor, are fitted at the two ends of the driving axle of 

 the cutting apparatus, that for hand driving being above the arm at 

 the level of the other pulleys of the cutting apparatus, and that for 

 the reception of the band from the motor being arranged below the 

 arm, at the level of the upper niche of the grinding pulley. A small 

 electric motor, driven by the current from three pint bichromate cells, 

 affords ample power for the purpose, and the speed of revolution is 

 best controlled by a friction brake upon the motor. 



The instrument has been constructed by Messrs. Troughton and 

 Simms, who are prepared to furnish copies of either this or the 

 smaller instrument. The author desires to express his thanks to 

 Mr. Skinner, of the Charlton Works, for invaluable assistance in 

 devising it, and to Mr. F. Chapman, of the geological laboratory of 



