185 



dating the causes of this striking discordance. The work done in this 

 field has chiefly led to explanations of the phenomena considered 

 from two different points of view. One class of investigators regards 

 the external form as the decisive and essential criterion for attributing 

 the right degree of symmetry to the crystal; and the discordance 

 between this symmetry and that of the optical phenomena observed 

 is explained by them by the supposition of the influence of secondary, 

 disturbing forces, like internal tensions produced by isomorphous 

 admixture, by rapid cooling, by changes in volume as a consequence 

 of polymorphic transformations, etc. The other view is, that the 

 optical properties reveal the true character of the space-lattice of 

 the crystal, and, therefore, of the true symmetry of the molecular 

 arrangement itself, while the external form is only to be considered as 

 a simulated, a mimetic one, exhibiting only an appparent symmetry. 

 According to the first view, the disparity mentioned above may 

 really be considered as an occurrence of "optical anomalies", while 

 according to the second, it is reduced rather to a case of "geometrical 

 anomalies" than to one of optical deviations. To the adherents of 

 the views first mentioned, objects of this kind are of a higher sym- 

 metry than from their optical behaviour they appear to be; for 

 the supporters of the last mentioned views, these crystals appear 

 higher symmetrical than they really are. 



8. On the other hand, the explanation of the rotatory power 

 of uniaxial crystals first discovered by Biot, has not been given 

 in any satisfactory way since the development of the optical theory 

 of that phenomenon by Fresnel. The latter had made the supposition, 

 that the propagating rectilinear ray consisted in reality of two equal 

 circularly polarised rays with opposite rotation-directions, of which 

 the one traversed the crystal with a greater speed than the 

 other. The result of this difference in velocity is a difference 

 in phase, and if the action of both rays on leaving the crystal 

 be again combined, a deviation of the original plane of polarisa- 

 tion, either to the right or to the left, must necessarily occur. 



This conception is, however, more a description of the phenomenon 

 than an explanation, because it includes no rational cause, either why 

 the one ray should be retarded in the crystalline medium with respect 

 to the other, or why the phenomenon, so far from being a general one 

 for such uniaxial crystals, is on the contrary a relatively rare one. l ) 



') For a review of the phenomena of circular polarisation, cf. : M. Berek, 

 Fortschr. f. Min., Kryst. u. Petrogr., Ed. G. Linck, 4, 73, (1914). 



