618 J. W. EVANS ON THE DETERMINATION OF xMINERALS UNDER 



cross wires and therefore to one of the directions of vibration in 

 the section (figs. 7, 8 and 17). A section showing a symmetrical 

 isogyre is itself said to be symmetrical. 



A symmetrical section is always cut at right angles to a plane 

 of optical symmetry, of which the central isogyre is the trace. 



Every section of a uniaxial mineral is cut at right angles to a 

 plane of optical symmetry, while this is only exceptionally the 

 case w T ith sections of biaxial crystals. If, therefore, every section 

 of a mineral in a rock section shows a symmetrical isogyre, we 

 may safely assume that the mineral is uniaxial. 



As a general rule in biaxial crystals a central isogyre is curved 

 and oblique to the cross wires (figs. 16, 19). 



A pseudosymmetric isogyre is straight, but is parallel not 

 to one of the cross wires, but to the line bisecting the angle 

 between them (fig. 18). 



A pseudosymmetric section is only met with in crystals 

 whose optic axial angle is 90 and the normal of such a section 

 lies in one of the planes containing the optic normal and one 

 of the optic axes of the crystal. 



If an isogyre is formed of two bars, but only one of these 

 passes through the centre of the field, the nature of the isogyre 

 and of the section is determined by the portion of the isogyre 

 which passes through the centre. 



If the two bars meet at right angles in the centre and form a 

 cross, they are both straight and parallel to the cross wires and 

 therefore symmetrical. The section must accordingly have been 

 cut at right angles to two planes of optical symmetry and to 

 the line of optical symmetry in which they meet. In a biaxial 

 crystal this line is either a bisectrix or the optic normal. In 

 the latter case, the cross is somewhat indistinct and in crystals 

 with an optic axial angle approaching a right angle it becomes 

 unrecognisable. If a section of a uniaxial crystal show a central 

 cross, it is either cut at right angles to the optic axis, and therefore 

 to an infinite number of planes of optical symmetry, or it is 

 parallel to the optic axis. In the latter case, again, the cross 

 is indistinct. 



The Movements of Isogyres. The movements of a symmetrical 

 isogyre, when the stage is rotated alternately in opposite directions, 

 are symmetrical to the cross wire to which it is parallel, while 

 those of a pseudosymmetric isogyre are symmetrical in the same 



