CRYSTALLINE FORMS. 99 



tion of the two lines is the greatest possible. The over- 

 lapping takes place in what is called the principal section of 

 the crystal, and the greatest separation occurs in a plane 

 very nearly coinciding with the equator of double refraction, 

 which is a plane at right angles to the crystallographic axis 

 of the rhomboid. 



If we cut off the summits of the rhomboid, and polish the 

 new faces, we shall find, that a pencil of light transmitted 

 through these new faces is not divided into two. The line 

 along which there is no double image, is named the axis of 

 double refraction, or the optic axis. 



This discovery confirms, in a very important degree, the 

 correctness of the arrangement proposed by Mohs. Sir 

 D. Brewster has ascertained that all minerals belonging to 

 the same system are connected by intimate optical relations; 

 for instance, that all those which belong to the rhombo- 

 hedral and pyramidal systems, have only one axis of double 

 refraction, which is coincident with the axis of symmetry of 

 the crystal ; that all minerals belonging to the prismatic 

 system, and its subordinates, have two axes of double refrac- 

 tion ; while all belonging to the tessular system have either 

 three axes in equilibria, or are otherwise so constituted 

 with reference to this property, that they do not display any 

 double refraction at all. 



The above imperfect sketch will- suffice to show that 

 crystallography is an extremely interesting and instructive 

 study, and one which can by no means be omitted in an 

 account of the external characters of minerals. At the 

 same time it cannot be denied, that however valuable or 

 attractive to the experienced mineralogist, it is of little 

 practical use in furnishing a guide for distinguishing minerals. 

 This is especially the case as regards the beginner and the 

 practical geologist. Comparatively few minerals occur in 

 the state of regular crystals ; they are met with, either mas- 

 sive or so imperfectly crystallised, that the student is com- 

 pelled to have recourse to other characters, which, when 

 united, are more obvious, though singly they may be less 

 so than that afforded by the measuring of crystals. Crys- 

 tallography, it may be said, is to mineralogy what mathe- 

 matics is to calculation ; while the general characters of 

 minerals may be considered as representing the arithmetic 



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