366 PLUCKER ON THE REPULSION OF THE OPTIC AXES 



phaenomena concerning binaxial crystals, as produced by diamag- 

 netisra and magnetism, the position of the two axes, the external 

 form of the crystal and the direction of the suspension as regards 

 these axes, I shall premise the following general observations. 



In a binaxial crystal three normal positions of suspension may 

 be distinguished, in three directions at right angles to each 

 other : — 



1. In a direction Z, which is at right angles to the plane of 

 the two optic axes. 



2. In the direction of the central line Y, between the two 

 optic axes. 



3. In the direction X, which is in the plane of the two optic 

 axes at right angles to their central line. 



In the first position, the plane X Y of the two optic axes 

 oscillates horizontally ; and, in consequence of the repulsion of 

 the axes, the central line Y becomes equatorial*. 



This action, — 



a, when the crystal is magnetic, and its dimension according 

 to the middle line Y is less than that at right angles to it, in the 

 direction X ; 



b, when it is diamagnetic, and its dimension in the direction 

 Y is greater than that in the direction X ; 



is in the one case increased by the magnetic attraction, in the 

 other by the diamagnetic repulsion of the mass. When, on the 

 other hand, 



* In the text we have implied that the inclination of the two axes to each 

 other is not great. Strictly speaking, according to our view, even in uniaxial 

 crystals, when suspended at right angles to the axis, the mere axial action pro- 

 duces a double position of equilibrium, one stable the other unstable. If 

 we admit that the single axis subdivides into two, which are equally distant 

 in the horizontal plane on each side from the original axis, so that this now be- 

 comes the middle line Y, the unstable equilibrium becomes completely stable ; 

 but its stability is at first slight, becoming greater when the two axes are more 

 opened ; and when these axes form with each other an angle of 90°, the stabi- 

 lity of the two positions of equilibrium becomes the same whether the crystal is 

 suspended in the direction X or Y. Probably if we gave a crystal of protosul- 

 phate of iron, the two optic axes of which are at right angles to one another, 

 such an external form that the magnetic attraction produced no rotation, it 

 would present two equally stable positions of equilibrium. In an experiment 

 with a thin circular (diamagnetic) plate of sulphate of lime about 18 millim. in 

 diameter, in the plane of which the two optic axes form an angle of 60°, I could 

 only with] difficulty retain it in the second less stable position of equilibrium. 

 The experiment must be repeated with a larger plate, or with the apices of tlie 

 poles more pointed ; at the same time it must be borne in mind, that when, in 

 consequence of the form, the diamagnetic directive force has not completely 

 vanished, it diminishes the stability of the equilibrium. 



