372 PLUCKER ON THE REPULSION' OF THE OPTIC AXES 



axial direction predominates, or where this is not the case ; 

 as, for instance, the optical condition of melted sugar which 

 shortly after solidification, comes under the latter, but after the 

 lapse of some time, under the former category. 



43. When a sphere (or even a rotating cylinder with its axis 

 vertical) is suspended between the two apices of the poles, we 

 do not find any rotation occur whether the mass be magnetic 

 or diamagnetic. The only action which can come into play in 

 this case, is the repulsion of the axes. If the sphere is made 

 from a uniaxial crystal, each time it is suspended the axis 

 becomes equatorially arranged. However, before it becomes 

 fixed in this position, it makes oscillations about it, the rapidity 

 of which is greater in proportion to the difference of the direction 

 of suspension from the direction of the axis. A pi-eliminary ex- 

 periment with a sphere of rock-crystal 57 millim. in diameter, 

 has shown that we may in this case obtain accurate admeasure- 

 ments*. 



If, in any two different positions of suspension, we mark on 

 the sphere the equatorial plane in each case, the section of the 

 two planes determines the axial direction of the crystal. 



In the last determination the external form is of no conse- 

 quence, provided we have convinced ourselves that in the mass 

 under examination the axial direction has overcome its mag- 

 netism or diamagnetism. 



Lastly, when the mass is made from a binaxial crystal, the 

 middle line between the two axes in this determination assumes 

 the place of the single axis. 



44. Faraday has already observed the modifications which 

 occur when a body is suspended between two surfaces of the 

 poles instead of the apices. As regards diamagnetic bodies, the 

 following experiment is characteristic. 



I placed upon each pole a parallelopipedal keeper 189 millim. 

 in length, so that the surfaces of the poles, which formed a right 

 angle 6/ millim. broad and 27 millim. in height, were directly 

 opposite each other at such a distance that a cylinder of bismuth 

 34 millim. long and 6 millim. thick could oscillate freely between 

 them. This cylinder of bismuth being thus suspended so that 



* I may remark here, that some other crystalline mass is best used for these 

 detennmations; for in i-ock-crystal the axial action was feeble beyond expecta- 

 tion, and the phienomenain several experiments were but slightly apparent, less 

 so than in any of the other experiments. 



