368 PLUCKER OX THE REPULSION OF THE OPTIC AXES 



Y and Z, that is, still indicating the corresponding dimension 

 by the same sign 



Z > Y > X. 

 The substance of arragonite is strongly diamagnetic. When 

 suspended in the direction Z, the crystal assumed such a posi- 

 tion that Y became equatorial ; and as Y > X, this action was 

 increased by the diamagnetic action exerted upon the mass of 

 the crystal. 



In consequence of the diamagnetic action, when the ciystal is 

 suspended so that it can oscillate with its longitudinal direction 

 Z horizontal, it rotates so that this direction becomes placed 

 at right angles to the line of the apices of the poles ; and the 

 force with which this is effected is evidently (neglecting magni- 

 tudes, which cannot here be taken into account) of the same in- 

 tensity, in tvhatever direction of the plane X Y the crystal is sus- 

 pended. However, in the two ordinary positions of suspension 

 in X and Y, if we admit the existence of a repulsive force, ex- 

 erted by the two poles upon the two axial directions, the action 

 exerted by it is very different as regards intensity. This can 

 in each case consist of a single rotation only of the two axial 

 directions around the alternate line of suspension, whence each 

 optic axis passing through any one point of the crystal describes a 

 one-sheeted hyperboloid of rotation, or when it especially cuts 

 the line of suspension, a conical surface. The moment of rota- 

 tion, however, on suspension in Y, is less than on suspension in 

 X ; and, in fact, the more so the more acute the angle which the 

 two optic axes form with each other, so that with this angle (if 

 the crystal be uniaxial) the former moment of rotation com- 

 pletely vanishes. Hence it follows that when, as in the experi- 

 ments with topaz and sugar (32, 33), the axial action, on re- 

 moving the apices of the poles, overcomes the diamagnetic 

 action, both of which (because Z>X and Z>Y) act in oppo- 

 site directions, this must ensue later when the suspension is in 

 the direction of the middle line Y than when in that of X. 



On approximating the apices of the poles as much as possible, 

 the crystal each time became placed like a diamagnetic body 

 with the longitudinal direction Z equatorial ; and on shortening 

 the silk thread, rotated on each occasion 90°. When the crystal 

 was suspended in the direction of the middle line Y, this took 

 place when the crystal was raised about 40 millim. above the line 



