has the direction of an approximately hexagonal or trigonal axis 

 of the space-lattice, and the rhombic crystal mentioned above is 

 a real pseudo-hexagonal or pseudo-trigonal limit-form. If a monoclinic 

 crystal has an angle (3 only slightly different from 90, while moreover 

 in its axial ratio a : b : c, one or two of the quotients are close to 

 unity, the said monoclinic crystal has a space-lattice which is dis- 

 tinctly pseudo -tetragonal or pseudo-cubic. 



Such pseudo-symmetrical crystals will often betray in their external 

 habit a certain approximation to higher symmetrical forms: thus 

 rubidium-nitrate has a strikingly simulative hexagonal aspect 1 ). 

 Or they will have a definite set of gliding-, or of cleavage-planes 

 which are nearly parallel to the faces of a crystalform with a 

 higher symmetry. 



Pseudo-symmetrical crystals therefore are such as closely simulate 

 a higher symmetry than they really have; this higher symmetrical 

 form is in many respects like an ideal model to which the proper 

 symmetry of the crystal tends, without ever reaching it. 



Now it is one of the most remarkable facts observed in inorganic 

 nature, that such pseudo-symmetrical crystals exhibit a strong and 

 undeniable tendency to increase their deceptive appearance yet more 

 by repeated twinning and regular aggregation. 



The new complexes thus produced are called mimetic forms; 

 the phenomenon itself bears the name of mimicry 2 ). This mimicry 

 is often so perfect that only a careful study of the optical and physi- 

 cal properties which reveal the true symmetry of the real molecular 

 arrangement of the crystal, can show the true character of it. As 

 therefore the optical properties are evidently often in discordance 

 with the geometrical form exhibited by these mimetic crystals, 

 they are said to manifest optical anomalies. To the discussion of 

 these anomalies we shall return later on. 



4. Some instances may elucidate this occurrence of mimetic forms. 

 Chrysoberyll is a beryllium-aluminate of .the composition: 

 BeO, ALO Z . 



The mineral is orthorhombic (a : b: c 0,4.701: i: 0,8500); but 

 its space-lattice is obviously pseudo-hexagonal, as is proved i. a. 

 by the fact that the angle (001) : (Oil) is 60 13'. Now three 



1) F. M. Jaeger, Zeits. f. Kryst. 43. 588. (1907). 



2) G. Tschermak, Lehrbuch der Mineralogie (1897), p. 91; O. Miigge, Neues 

 Jahrb. f. Miner. Beil. Bd. 14. 245. (1901); 16. 335. (1903); P. Fischer, Disser- 

 tation, Gottingen, (1911); V. Goldschmidt, Zeits. f. Kryst. 43. 353. (1907). 



