180 



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 apparent 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 higher sym- 

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

 the supporters of the last mentioned views, these crystals appear 

 higher symmetrical than they really are. In the first case we 

 should have to look on them as on human beings who, by a com- 

 bination of unfavorable circumstances, are forced to show themselves 

 in a degenerate state and worse than they ought to be ; in the second 

 case we should have to regard them more as we should a servant, 

 who dressed himself in the clothes of his master and with the external 

 signs of his dignity, giving himself the ridiculous appearance of an 

 individual whose exterior is in flagrant contrast with his inner 

 inferiority. If this matter might be looked at in this anthropomorphic 

 way, the whole existence of these strange individuals would have 

 to be considered in the first case as an almost tragical fact, in the 

 second rather as a caprice of nature full of humour. 



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 ag^in combined, a deviation of the original plane of polarisa- 



