106 LEPIDOLITE 



effect, such as the production of motion which shall resemble as nearly as possible the 

 kind of stroke which would be given by the hand, these tools are eventually brought 

 to true spherical figures which fit each other exactly. 



The glasses for lenses, being selected of suitable quality, are brought to a cir- 

 cular form by means of flat pliers called shanks. The pressure of the pliers 

 applied near the edges of the glass causes it to crumble away in small frag- 

 ments, and this process, which is called shanking or nibbling, is continued until the 

 glasses are made circular, and of a little larger diameter than the finished size of 

 the lenses. 



A cement is made by mixing wood-ashes with melted pitch. Some nicety is re- 

 quired in the adjustment of the proportion, since the cement must not be too adhesive, 

 nor must it be too hard or too brittle ; generally about 4 Ibs. of wood-ashes to 14 Ibs, 

 of pitch are employed. This when melted is poured on one side of the glasses to be 

 ground, in small quantities at a time, until a sufficient quantity adheres to the back 

 of the lens to form a handle. The glass is rough ground by rubbing it within the 

 spherical shell. The glass is rubbed with large circular strokes, and the shell is 

 usually placed within a shallow tray to catch the loose emery or polishing powder 

 which may be employed. When one side is rough ground in this way, the glass is 

 warmed to detach it from the handle, which is transferred to the other side and the 

 operation repeated. When both sides are thus rudely formed, the lenses are cemented 

 upon a runner. The best object-glasses for telescopes are 

 ground and polished singly, while as many as four dozen of 

 common spectacle glasses are ground and polished together. 

 When many are thus fixed on one runner, the number must 



be such as will admit of their being arranged symmetrically 



around a central lens, as 7, 13, or 21 ; or sometimes 4, form 



the nucleus, and then the numbers run 14, 30. Lenses of ordinary quality are usually 

 ground true and polished 7 at a time. This runner with its lenses attached is shown 

 in Jig. 1383. 



The cement at the back of the lenses is first flattened with a heated iron. The cast- 

 iron runner is heated just sufficiently to melt the cement, and carefully placed upon 

 the cemented backs of the lenses. As soon as the cement is sufficiently softened to 

 adhere firmly to the runner, it is coated with a wet sponge, as the cement must only 

 be so far fused as to fill up the spaces nearly, but not quite, level with the surface of 

 the lenses. The block of lenses is now mounted upon a post, and ground with the 

 concave brass tool, fig. 1382, motion being given to it either by the hand or by ma- 

 chinery similar to the sweeping motion already named. As the grinding proceeds, 

 the fineness of the emery-powder employed is increased, until in the last operation it 

 is sufficiently fine to produce a finely-polished surface. This grinding being com- 

 pleted successfully, the lenses have to be polished. The polisher is made by warming 

 a cast-iron shell, and coating it uniformly about one quarter of an inch thick with 

 melted cement. A piece of thick woollen cloth is cut to the size of the polisher and 

 secured to it, and pressed into form by working the brass tool within it. When 

 this is properly adjusted it is covered with very finely-divided putty-powder, sprinkled 

 with a little water, and the powder worked into the pores of the cloth with the brass 

 convex tool. Kepeated supplies of putty-powder are put on the polisher until it is 

 made quite level, and it is worked smooth with the tool. Many hours are expended 

 in the proper preparation of a polisher. When completed it is fixed upon the block 

 of lenses still fixed to the post, and worked with wide and narrow elliptical strokes. 

 Where a very large number of glasses are ground or polished at the same time, this 

 peculiar motion is imitated by the excentric movement of a lever attached to the 

 revolving shaft. In the processes of grinding and polishing, other materials beside 

 emery and putty-powder are sometimes employed, such as raddle, an earthy oxide of 

 iron, the finer kinds of which are much employed in the large lens manufactory at 

 Sheffield. 



The best account of these processes and of the instruments used is by the late 

 Andrew Ross, in the fifty-third volume of 'the Transactions of the Society of Arts. In 

 HoltzapffeVs Mechanical Manipulation there is also some practical information. See 

 PHOTOGRAPHY. 



XiENTXXiS. The seeds of Eroum lens, a leguminous plant, from which the flour 

 called Ervalenta, or Revalenta, is prepared. 



XiSPXBXirz:. C 10 H 9 N (C^ 9 !*). A volatile base, homologous with chinoline, 

 found in coal-naphtha and in the fluid produced by distilling cinchonine with 

 potash. 



lEPlDOCROClTE. A scaly or fibrous variety of Gothite, or hydrous peroxide 

 of iron. See IRON. 



XiEPXDOXiXTE, or Lithia Mica. A beautiful purple mineral, which occurs in 



