122 SCIENCE PROGRESS. 



offered explanations of their so-called optical anomalies. 

 Some of these crystals when examined in thin sections in 

 the polarising microscope are seen to possess a weak 

 birefringence and to be partitioned into symmetrical areas, 

 quadrants or sextants for example, in which the light may 

 be extinguished uniformly over alternate areas successively 

 as the crystal is rotated between the crossed Nicol prisms, 

 and not over the whole section as in an ordinary crystal. 

 Alum, Garnet and Boracite are familiar examples. 



Other apparently cubic crystals which exhibit optical 

 anomalies present the regular features of uniaxial and 

 biaxial crystals belonging to other systems, the orientation 

 of the optic axis, or of the optic axes, being different in the 

 adjacent symmetrical partitions. 



Neumann, Klocke, Klein and others have attributed the 

 birefringence to strains set up either during crystallisation 

 or during fall of temperature and resulting contraction after 

 they have solidified ; Mallard on the other hand propounded 

 the theory that such crystals are pseudosym metrical and 

 consist of a number of uniaxial or biaxial individuals pos- 

 sessing less than cubic symmetry, twinned together so as to 

 simulate the symmetry of the cube. 



Both processes are illustrated by certain well-known 

 examples. There are without doubt crystals belonging 

 to the orthorhombic or monosymmetric systems which 

 do aggregate themselves by twinning and constitute a 

 complex crystal of apparently higher symmetry than their 

 own,, and it is equally true that birefringence may be pro- 

 duced by pressure both in cubic crystals and in uncrystalline 

 substances ; it is even possible to make gelatine moulds in 

 the form of crystals, or of other geometrical solids, which 

 perfectly simulate the optical characters of a uniaxial or 

 biaxial crystal, and when examined in convergent polarised 

 light show the familiar coloured circles or lemniscates char- 

 acteristic of a crystal. 



Lehmann, however, dismisses the possibility of his 

 liquid crystals being merely very soft solid bodies, whose 

 birefringence is produced by one or other of these causes 

 or by the inclusion of some impurity, on the following 



