DIAMOND: TECHNICAL APPLICATIONS 245 



entrusted to the cutters there, who are held to be the most skilful and who best 

 understand how to provide quite small stones with regularly arranged facets. 



While the softer precious stones are often submitted to the operation of engraving, 

 the diamond is very rarely so treated. Whether diamonds were engraved at all by the 

 ancients is a matter open to grave doubt ; only a few engraved diamonds belonging to more 

 recent times are known ; one of these bears the portrait of Don Carlos, and another was 

 engraved with the Spanish arms for Charles V. The engraving of the diamond is never 

 attempted at the present day, since the result of such attempts is in no way commensurate 

 with the labour expended. That the art was not entirely unknown in the Orient is 

 evidenced by the existence of a beautiful Indian octahedron of 30 carats, described by 

 Boutan, one face of which was engraved with a devotional motto. According to G. Rose's 

 account, some of the faces of the irregularly-formed diamond known as the " Shah," now in 

 the Russian crown jewels, were engraved in Persian characters with the names of Persian 

 kings. Two of the faces of the " Akbar Shah," another large diamond, were engraved with 

 Arabic inscriptions, but these, like the engravings on the " Shah," had to be sacrificed when 

 the stone was re-cut. 



Only rarely have diamonds been bored and threaded as beads ; the art is said to be 

 practised at the present day at Ghent and Venice, in the latter city being the last remains 

 of an old diamond-cutting industry which once flourished there. The boring is effected by 

 the use of diamond powder, the perforation being started with a diamond point and 

 continued with a steel point charged with diamond dust. 



3. TECHNICAL APPLICATIONS. 



The value of the diamond in its technical applications depends, as a rule, upon its 

 exceptional hardness, and only in a few cases on its high refractive index. 



The high index of refraction of the diamond led to an attempt to utilise this substance 

 for the construction of microscope objectives. A diamond lens having only a slight 

 curvature will give the same magnification as a lens of much greater curvature constructed 

 of some less strongly refracting substance, such as glass. The disadvantages connected with 

 the use of strongly curved lenses would be thus avoided, and, moreover, diamond lenses on 

 account of their hardness would not be liable to be scratched by particles of dust and other 

 matter. Experiments in this direction were made mainly by Pritchard, under the direction 

 of Dr. Goring, between 1824 and 1826 ; although Pritchard was successful in preparing a 

 few suitable lenses their use has never been adopted, probably on account of the difficulties 

 connected with their construction and the prohibitive cost of the material. 



Very extensive use is made of the diamond for the purpose of cutting glass, every 

 glazier being in the possession of a tool known as a glazier's diamond. The stone set 

 in this tool must be bounded by two rounded crystal-faces meeting in the curved cutting 

 edge, which should not be too obtuse. This cutting edge is drawn with a slight pressure 

 over the surface of the glass to be cut, and produces a fine scratch, not more than ^-dtt iii<^h 

 in depth, but which is sufficient to cause the glass to be easily broken in the direction of 

 the scratch. The cutting edge, when properly used, cuts into the surface layer of glass like 

 a wedge ; when drawn across the glass by an unpractised hand, it simply tears instead of 

 cutting the surface, and this also happens when a sharp pointed splinter of diamond is used 

 instead of the curved cutting edge. In the direction of the jagged furrow produced by such 

 means glass will not break as it does along a clean cut. Wollaston investigated this matter 

 in detail, and found that the edge formed by the intersection of two curved natural faces of 



