332 HISTORY OF MINERALOGY. 



history of this very rich and beautiful department of science, we have 

 already given some account, in speaking of Optics. The first facts 

 which were noticed, those relating to double refraction, belonged ex- 

 clusively to crystals of the rhombohedral system. The splendid phe- 

 nomena of the rings and lernniscates produced by dipolarizing crystals, 

 were afterwards discovered ; and these were, in 1817, classified by Sii 

 David Brewster, according to the crystalline forms to which they 

 belong. This classification, on comparison with the distinction of 

 Systems of Crystallization, resolved itself into a necessary relation of 

 mathematical symmetry : all crystals of the pyramidal and rhombohe- 

 dral systems, which from their geometrical character have a single axis 

 of symmetry, are also optically uniaxal, and produce by dipolarization 

 circular rings ; while the prismatic system, which has no such single 

 axis, but three unequal axes of symmetry, is optically biaxaJ, gives lem- 

 niscates by dipolarized light, and according to Fresnel's theory, has 

 three rectangular axes of unequal elasticity. 



[2nd Ed.] [I have placed Sir David Brewster's arrangement of crys 

 talline forms in this chapter, as an event belonging to the confirmation 

 of the distinctions of forms introduced by Weiss and Mohs ; because 

 that arrangement was established, not on crystallographical, but on 

 optical grounds. But Sir David Brewster's optical discovery was a 

 much greater step in science than the systems of the two German crys- 

 tallographers ; and even in respect to the crystallographical principle, 

 Sir D. Brewster had an independent share in the discovery. He 

 divided crystalline forms into three classes, enumerating the Hainan 

 " primitive forms" which belonged to each ; and as he found some ex- 

 ceptions to this classification, (such as idocrase, &c.,) he ventured to 

 pronounce that in those substances the received primitive forms were 

 probably erroneous ; a judgment which was soon confirmed by a 

 closer crystallographical scrutiny. He also showed his perception of 

 the mineralogical importance of his discovery by publishing it, not only 

 in the Phil. Trans. (1818), but also in the Transactions of the Wer- 

 nerian Society of Natural History. In a second paper inserted in 

 this later series, read in 1820, he further notices Mohs's System of Crys- 

 tallography, which had then recently appeared, and points out its 

 agreement with his o\yv . 



Another reason why I do not make his great optical discovery a 

 cardinal point in the history of crystallography is, that as a crystallo- 

 graphical system it is incomplete. Although AVC are thus led to dis- 

 .inguish the tessnlar and the 2)rismatic systems (using Mohs's terms) 



