360 PLiJCKER ON THE REPULSION OF THE OPTIC AXES 



this, regard had first to be paid to distinguishing whether the 

 substance of the body was magnetic or diamagnetic. It was spe- 

 cially important to test a crystal of some diamagnetic substance 

 as well as tourmaline, which derives its magnetism from the iron 

 contained in it. To obtain an effect which could in nowise be 

 ascribed to diamagnetism, a crystal of such form must be selected, 

 or such a shape must be given to it by artificial means, that its 

 shortest dimensions coincide with the direction of the axis. The 

 form of the colourless diamagnetic tourmaline mentioned in para- 

 graph 13 was not adapted for the purpose of a decisive experi- 

 ment. 



19. I therefore first examined calcareous spar, the substance 

 of which is decidedly diamagnetic. A colourless crystal, bounded 

 by natural faces of cleavage, the length of the angles of which 

 were 60 millim., 50 millim. and 28 millim., was suspended, 

 without using the apices of the poles, in such a manner that 

 its axis could oscillate horizontally between the poles. This 

 axis became placed exactly equatorially, whereby the crystal as- 

 sumed a position in which neither a magnetic nor a diamagnetic 

 mass of the same form would have rested when acted upon by 

 the magnetic and diamagnetic action of the poles of the electro- 

 magnet. 



20. I then took a smaller crystal of the primary form, which, 

 overcoming the diamagnetism, arranged itself between the ap- 

 proximated apices of the poles, so that its axial direction became 

 exactly equatorial. 



A second such crystal, but larger, the length of the angles of 

 which was 15 millim., and the obtuse angles of which were 

 ground off perpendicularly to the optic axis to such an extent that 

 the thickness of the crystal in its axial direction was 10 millim. 

 only, was suspended in the same manner as the two former, 

 and assumed the same position, as regards its axis, as those. 

 When, however, the apices of the poles were so far approxi- 

 mated, that on account of its large dimensions it could no longer 

 remain in the Ime of the apices of the poles, it rotated 90°, as- 

 suming the same position as a diamagnetic body, so that its axial 

 direction coincided with the line of the apices of the poles, and 

 the diamagnetic repulsion of the mass determined the position 

 of the crystal. 



21. I then examined several other plates cut perpendicularly 

 to the axis, all of which exhibited the same phaenomenon. One 



