THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 619 



way to the diagonal to which it is parallel. If the movements 

 are unsymmetrical, the isogyre must be so likewise. 



If, when the stage is rotated, one of the ends of an isogyre 

 at the boundary of the field moves round the circumference 

 in the same cyclical direction as that in which the stage is 

 rotated, that end is said to be proximal. If it moves round 

 in the opposite direction or is stationary, it is distal. The 

 terms homodrom and antidrom are used by Becke, but they 

 are misleading, if a microscope with rotating nicols be employed, 

 and proximal and distal are accordingly more suited for general 

 use. The manner in which they are applied is illustrated in 

 figs. 7, 9-11 and 16-18. 



An isogyre consisting of a single band has usually one end 

 proximal and the other distal. A proximal end is directed 

 towards the nearest optic axis, or, if it be practically equidistant 

 from the two optic axes, to the nearest bisectrix. 



An isogyre consisting of two bars intersecting in a cross 

 has in biaxial crystals (fig. 13) two proximal ends opposite to 

 each other and two distal ends. If the centre of the cross repre- 

 sents a bisectrix, the proximal ends are directed towards the 

 optic axes and the bar to which they belong marks the trace of the 

 optic axial plane. The distal ends lie in the direction of the optic 

 normal. If the section is, on the other hand, cut at right angles to 

 the optic normal, the proximal ends point to the acute bisectrix 

 and the distal towards the obtuse bisectrix. 



On rotation of the stage the cross breaks up into two hyper- 

 bolic branches, each with one proximal and one distal end. These 

 move away from the centre and may pass entirely out of the field. 

 In sections of uniaxial crystals cut at right angles to the optic 

 axis (fig. 21) the cross does not break up on the stage being rotated, 

 and the ends are stationary and therefore distal. The phenomena 

 :n sections parallel to the optic axis are similar to those in 

 sections at right angles to the optic normal in biaxial crystals. 

 The proximal ends are directed towards the optic axis, while 

 the bar with distal ends lies in the plane of optical symmetry 

 at right angles to the optic axis. 



If the distal end of an isogyre move more rapidly than the 

 proximal end, the movement may be compared to that of a 

 pendulum. This happens when the proximal end is directed 

 towards an optic axis. If the section is symmetrical, either 



