'82 H. J. Grayson : 



grooves appeared on the area of contact with the screw facet, which 

 led to their rejection. Topaz, \\hich is only a trifle harder than 

 quartz, and not so tough as the variety of quartz known as agate, 

 likewise failed, and probably from the same cause as tlie latter. 



With respect to sapphire, this was actually one of the fii'st sub- 

 stances made use of, and was only temporarily discarded because of 

 the difficulty of working up a bearing face to the recpiisite perfec- 

 tion. A second attempt to prepare a fine crystal slab of this gem, 

 after the rejection of steel, agate and topaz, was rewarded with a 

 greater n)easure of success; the result from the second attempt being 

 a finely polished optically true surface, over a central circular area 

 of the crystal § in. in diameter. This face, which was free from 

 defects, either of its crystalline structure or tlmse arising from the 

 process of woi-king it, was formed on the face of a S(|uare slab of 

 sapphire f in. in diameter and under 3-16 in. thick. Two sapphire 

 thrust plates have been successfully prepared. One of them, which 

 is circular in shape, has been cut f)'om a synthetically formed crystal 

 of sapphire. The latter is somewhat easier to work than the natural 

 gem, probably because the crystalline cleavages are less developed. 

 The artificial gem, however, has tlie disadvantage of being less 

 perfectly annealed, and, in consequence, requires greater care in 

 working to avoid fracture. The following outline of tlie process of 

 working a true sapphire thrust face may be worth recording, as it 

 differs from the lapidary's method of Avorking such gem faces. 



It is necessary in the first place to secure a good sapj^hire gem 

 .stone, its colour is immaterial; it should, however, be fairly large 

 and quite free from cleavage fractures and other similar defects. 

 The work of slicing and roughly cutting to shape is done after the 

 method followed by the lapidary; the grinding and polishing of 

 its faces requires more exact treatment than he usually gives to 

 these operations. In order to obtain good surfaces, truly worked 

 and free from scratches or other defects, three small laps were 

 first prepared and ground true on one face. One of these laps, 

 made of gun metal, was used, Avith finely crushed and separated 

 diamond powder, to grind up the faces and edges of the sapphire 

 plates until they were fairly smooth and parallel. A second lap, of 

 steel in this case, with still finer diamond powder, sufficed to finish 

 off one ground face until it was free from the scratches and pitting 

 remaining from the preceding grinding. A third lap of pure tin, 

 with a carefully prepared face, was now u.sed for polishing the 

 already optically true face of the crystal; the polishing material 



