370 PLUCKER ON THE REPULSION OP THE OPTIC AXES 



plicable anomaly, would now yield a beautiful confirmation of 

 my theoretical views. 



The crystal of staurolite was ti-ansparent and strongly mag- 

 netic. It formed a prism 18 millim. in length, the transverse 

 section of which was an irregular hexagon, A, B, C, D, E, F. 

 The opposite parallel lateral surfaces were about the same di- 

 stance apart ; this was 6 millim. The apices of the poles were 

 approximated as much as possible, and in all the experiments 

 retained undisturbed in the same position. The crystal was 

 suspended, at first perpendicularly to the lateral surfaces A, F 

 and C. D, secondly perpendicularly to the lateral surfaces A, B 

 and D, E. In both cases an elevation of exactly 25 millim. 

 above the line of apices of the poles was requisite to cause the 

 crystal to become equatorial. The crystal was then, thirdly, 

 suspended at right angles to the lateral surfaces B, C and E, F, 

 and the crystal could now no longer be rotated 90° with the 

 strength of the current used ; when raised 100 millim., it con- 

 tinued to maintain at least its axial position. Two other suspen- 

 sions were then made, the fourth in the direction of the line of 

 subdivision of the angle at A, which was at right angles to the 

 line of the third suspension, and the fifth in the direction of the 

 line of subdivision of the angle at B, In the fourth suspension, the 

 crystal, when raised 23 millim., was rotated 55 millim. ; in the 

 fifth, at an elevation of 50 millim., to the same point. Thus the 

 greatest moment of rotation produced by the repulsion of the optic 

 axes obtained in the fourth, and the least in the fifth position 

 of suspension. We thus di'aw the conclusion, that the general 

 direction denoted by X subdivides the angle at A, whilst the 

 direction Z is perpendicular to the lateral surfaces B, C and E, F. 



The angles of the prism were next measured, and the angles at 

 A and D found to be about 129°. But the primary form, as is 

 well known, is a right rhombic prism, in which the obtuse angles 

 are equal to the one measured. The plane of the optic axes 

 thus passes through the two obtuse angles of the primary form. 

 The two surfaces B, C and D, F, by which the two acute angles 

 are truncated, are parallel to the plane of the two optic axes. 



The above supposition, that the axis of the prism is the middle 

 line between the two optic axes, might require confirmation. 

 For this purpose I suspended the prism of staurolite longitu- 

 dinally ; it became placed so that the plane passing through the 



