620 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



it is at right angles to the optic axial plane of a biaxial crystal, 

 or it is a section of a uniaxial crystal, which makes a very 

 large angle with the basal plane. 



If both ends move at nearly the same rate, the isogyre passes 

 straight across the field, maintaining approximately its rectilinear 

 form and keeping parallel to one of the cross wires (fig. 11). This 

 is the case in sections of uniaxial crystals which make only a 

 moderate angle with the basal plane. It is not the only move- 

 ment occurring in isogyres of uniaxial minerals, as is commonly 

 supposed to be the case. 



Longitudinal and Transverse Directions. A direction of vibra- 

 tion (or extinction) is said to be longitudinal, when it is parallel 

 to the central isogyre where it passes through the centre, or 

 when it makes an angle of . less than 45 with it. The transverse 

 direction of vibration is that at right angles to the longitudinal 

 direction. If a central isogyre is diagonal, that is to say if it 

 bisects the angle between the cross wires, the directions of 

 vibration are neither longitudinal nor transverse: 



The ends of a longitudinal direction nearest to the proximal 

 <and distal ends of the isogyre may themselves be described as 

 proximal and distal respectively. 



In a central cross which breaks up into hyperbolic branches 

 when the stage is rotated, the horizontal or vertical bar with two 

 proximal ends marks the longitudinal direction, and in the 

 diagonal position becomes the axis of the hyperbola, if one be 

 visible (figs. 13 and 14). 



If there be a central cross which does not break up, the section 

 is, as already stated, cut at right angles to the optic axis of a 

 uniaxial crystal, and all directions are longitudinal (fig. 21). 



The Character of Isogyres and Sections. The character (sign) 

 of a central isogyre is that of the longitudinal direction (see p. 605), 

 and the same character is attributed to the section itself. A 

 diagonal isogyre and its section are said to be neutral, for they 

 /can be neither fast nor slow, since there is no distinction between 

 longitudinal and transverse directions. 



The determination of the character of sections in a rock-slice 

 enables us to form a conclusion as to the character of the mineral 

 as a whole. 



In a uniaxial crystal the character of all sections is the same 

 .as the character of the mineral, which is that of its optic axis. 



