Magneto-optic Properties of Crystals, 27 



may denote a line of contact very different from that denoted 

 by the short one ; and the line at right angles to the diagonals, 

 that is, the axis of the prism, a contact very different from 

 both. We can compare these lines two at a time. By hang- 

 ing the short diagonal vertical in the magnetic field, its rota- 

 tory power is annulled, and we can compare the long diagonal 

 and the axis. By hanging the long diagonal vertical, we can 

 compare the short diagonal and the axis. By hanging the 

 axis vertical, we can compare the two diagonals. From this 

 point of view the deportment of heavy spar and ccelestine, so 

 utterly irreconcileable with the assumption of an optic axis 

 force, presents no difficulty. If we suppose the proximity 

 along the axis of the prism to be intermediate between the 

 proximities along the two diagonals, the action of both cry- 

 stals follows as a necessary consequence. Suspended from 

 one angle, the axis must stand from pole to pole ; from the 

 other angle, it must stand equatorial. 



A ball of dough, made from bismuth powder, was placed 

 between two bits of glass and pressed to the thickness of a 

 quarter of an inch. It was then set edgeways between the 

 plates and pressed again, but not so strongly as in the former 

 case. A model of heavy spar was cut from the mass, so that 

 the shorter diagonal of its rhombic base coincided with the 

 line of greatest compression; the axis of the model with the 

 direction of less compression, and the longer diagonal of the 

 base with that direction in which no pressure had been ex- 

 erted. When this model was dried and suspended in the mag- 

 netic fields there was no recognizable difference between its de- 

 portment and that of heavy spar. 



When a crystal cleaves symmetrically in several planes, all 

 parallel to the same straight line, and, at the same time, in a 

 direction perpendicular to this line, then the latter cleavage, 

 if it be more eminent than the former, may be expected to 

 predominate ; but when the cleavages are oblique to each other, 

 the united action of several minor cleavages may be such as 

 to overcome the principal one, or so to modify it that its action 

 is not at all the same as that of a cleavage of the same value 

 unintersected by others. A complex action among the par- 

 ticles of the crystal itself may contribute to this result, and 

 possibly in some cases modify even the influence of proximity. 

 If we hang a magnetic body between the poles, it always shows 

 a preference for edges and corners, and will spring to a point 

 much more readily than to a surface. Diamagnetic bodies 

 will recede from edges and corners. The fluid is, as it werej, 

 discharged with greater power from a point. A similar action 

 among the crystalline particles may possibly bring about the 

 modification we have hinted at. 



