196 



was able to bring complete experimental evidence of this by the 

 aid of R on t gen- diagrams obtained with complex mica-piles, the 

 lamellae of which were crossing under angles of 45, 60, or 90, 

 and arranged in definite ways. As is wellknown, the plane of 

 the optical axes of muscovite is perpendicular to its plane of 

 symmetry (010), while at the same time it is almost perfectly 

 perpendicular to the plane of cleavage (001) of this monoclinic 

 mineral. If, for the purpose of preparing circularly polarizing 

 mica-piles, these lamellae now are piled up, say under angles of 

 60, - - it makes no sensible difference with respect to the optical 

 behaviour of the complex, whether the successive lamellae be all 

 rigorously parallel to each other, or some of them be rotated through 

 1 80 round an axis coinciding with the direction of the intersection 

 of the axial plane and the plane of cleavage (001). But with respect 

 to the final effect of the diffraction of the Ron t gen-rays in such 

 piles, this rotation through 180 appears to be not at all indifferent. 

 According to the particular way of piling up, be it in the case men- 

 tioned before, in triades or hexades of parallel or inversed lamellae, 

 it may be foreseen, that Rontgenograms of hexagonal, trigonal, 

 and even of only monoclinic symmetry can be obtained. The ex- 

 periments have fully confirmed these deductions; and they have 

 proved that, if amongst the numerous crossing, lower symmetrical 

 lamellae of polysynthetic twins, there is an only relatively small 

 number of them accidentally rotated through 180 round an axis 

 not coinciding with any direction of symmetry 'of the lamella, the 

 effect must necessarily be such, that a Rontgenogram of the 

 pseudosymmetrical complex under consideration appears to possess 

 e.g. only bilateral symmetry, while no optical disturbances or irre- 

 gularities whatsoever can be detected, even by the most sensitive 

 methods. Of some of these substances, e. g. of benitoite, other investi- 

 gators later have really obtained completely normal diffraction- 

 images; cf.: F. Rin-ne, Centr. Bi. f.Min.,(1919),p. 193. In the opinion 

 of the present writer, therefore, there is scarcely room for doubt, 

 that the explanation given by Mallard will prove to hold in the 

 larger number of cases 1 ). 



1) G. W. Wulff, Zeits. f. Kryst., 17, 592, (1890); G. Wyrouboff, Bull, de la 

 Soc. Miner., 8, 78, 398, (1885); 13, 213, 277, (1890); 14, 215, 233, (1891); Ann. de 

 Chim. et Phys., (6), 8, 340, (1886); etc.; F. Wallerant, Bull, de la Soc. Miner., 

 24, 155, (1901); 27, 184, (1904). 



