CRYSTALL GRAPH Y MINERALOGY. 3 1 3 



The admirable researches of Grailich and Lang, in Vienna, and 

 of Descloizeaux, in Paris, have given to the varieties of dispersion 

 just alluded to very important applications in crystallographic 

 work, and by this means errors into which crystallographers had 

 fallen regarding the systems to which some crystals belong have 

 been corrected. 



It is obvious, then, that the impulse thus given to crystallo- 

 graphic physics, and especially the optical section of physics, insti- 

 tuted the necessity for instruments capable of dealing with these 

 problems with considerable precision. The objects of the optical 

 instruments thus required may in general be classed as, 



1. The determination of the optical (uniaxal or biaxal, negative 

 or positive) characters of crystals. 



2. The determining the position in them of the optical principal 

 sections, and of the optic axes. 



3. The character of the dispersion of these axes for different 

 colours; and, 



4. The quantitative determination of the refractive indices, and 

 so of the optical elasticity of the crystal along its principal axes. 



As a step preliminary to the use of such instruments, the crystal 

 has to be prepared by the operations of the lapidary in cases 

 where the natural faces do not furnish the surfaces required. A 

 fine saw and a common file, hone-stones with buffs or rubbers 

 offering various kinds of surface, serve to give the requisite shape 

 to sections of such softer crystals as are formed in the laboratory 

 and may be met with in the mineral kingdom. But for the more 

 refractory minerals the instruments of the lapidary have to 

 be used ; these are discs of soft iron or steel charged with 

 diamond or emery dust for slitting the crystal, and wheels of 

 pewter charged with coarse emery or diamond dust ; plates of 

 various metals, but usually one of brass paved with the finest 

 flour of emery, for working the sections up to a surface, on 

 which the more delicate work of thinning down and finally of 



