LENS 105 



an agreeable taste. These are the ' sapucaya ' nuts of the fruiterers' shops. The 

 Brazil-nuts are the produce of the juvia tree (Bertholletia excelsa). See BRAZIL 

 NUTS. 



XiEDtTM PAXitTSTRE. This plant is employed in Russia to tan the skins of 

 goats, calves, and sheep, into a reddish leather of an agreeable smell ; as also in 

 the preparation of the oil of birch, for making what is commonly called Russia 

 leather. 



IiEER. An arched building, forming an annealing furnace, in which glass is 

 tempered or annealed. 



liEGUMlN'E. A name applied to vegetable casein, in allusion to its occurrence 

 in the seeds of many of the Leguminosce, or Pea and Bean family. 



XiEMXTXAlU' EARTH. A yellowish-grey earth, obtained from Lemnos by the 

 Greeks. It is very similar to fuller's earth. 



XiEMOirs. The fruit of the Citrus limonum. Both the juice and the peel of the 

 fruit are employed medicinally, and in the preparation of lemonade, See CITEIC 

 ACID, and OILS, ESSENTIAL. 



XiEiyxON" CRASS. The Andropogon citratum (De Can.). This, and certain allied 

 species, yield fragrant essential oils imported from India under the name of lemon 

 grass and citronelle oils. 



XiEMTS. (Lentille, Fr. ; Linsenglas, Ger.) Lenses are transparent bodies, usually 

 made of glass, which by their curvature either concentrate or disperse the rays of 

 light. Lenses are of the following kinds : Double convex : having the same or a 

 different degree of convexity on either side. Plano-convex, having one plane and one 

 convex surface. Concavo-convex, having one concave and one convex side, commonly 

 called meniscus lenses. Plano-concave, having one plane surface and one concave one; 

 and the double concave lens. 



The first three, which are thicker in the middle than at the edge, are converging 

 lenses, because they occasion the rays of light to converge in passing through 

 them. The others, which are thicker at the edges than in the middle, and there- 

 fore cause the pencils of light refracted through them to diverge, are called diverging 

 lenses. 



For the most complete examination of the laws regulating the construction of 

 lenses, and the action of these on the rays of light, we must refer the reader to Sir 

 John Herschel's admirable treatise on Light in the Encyclopedia Metropolitana. In 

 this work we have only to deal with the mode of manufacturing the ordinary varieties. 

 The spherical surfaces are produced by grinding them in counterpart tools, or discs 

 of metal, prepared to the same curvature as the lenses. For the formation of the 

 grinding tools, a concave and a convex template are first made to the radius of the 

 curvature of the required lens. The templates of large radius are sometimes cut 

 out of crown glass. More usually the templates are made out of sheet brass, the 

 templates of long radii are cut with a strong radius bar and cutter, and those of 

 only a few inches radius are cut in the turning-lathe. The brass concave and 

 convex gauges are cut at separate operations, as it is necessary to adjust the 

 radius to compensate for the thickness of the cutter, and the brass templates 

 are not usually corrected by grinding, as practically it is found more convenient 

 to fit the tools themselves together. The templates, having been made of the re- 

 quired radius, are used for the preparation of the grinding and polishing tools, 

 which for concave lenses consist of a concave rough grinding-tool of cast iron, called 

 a shell. 



A pair of brass tools is, however, the most important part of the apparatus. One 

 of these is concave and the other convex, made exactly to the curvature of the tem- 

 plates, and to fit each other as accurately as possible. 

 The concave tool is used as the grinder for correcting the 

 curvature of the lenses after they have been roughly 

 figured in the concave shell, and the convex tool is em- 

 ployed for producing and maintaining the true form of 

 the concave grinding-tool itself, and also that of the 

 polisher. These polishers are adjusted with great accu- 

 racy. The concave tool is placed upon the convex, and 

 they are first rubbed together dry, so that by the 

 brightened parts the inequalities may be distinguished ; 

 they are then ground true, first by means of emery and 

 water, and then with dry emery. 



The annexed figure (1382) represents those tools, 

 which are fitted with screws at the back, so that they can be fixed upon pillars ifl 

 connection with the machinery for giving motion to them. 



By grinding with sundry niceties of motion, which are required to produce the best 



