364 PLUCKER ON THE REPULSION OF THE OPTIC AXES 



land, which was cut in such a manner that it approximated in 

 form to a right rhombic prism, with its two pairs of opposite 

 lateral surfaces perpendicular to the two optic axes. The length 

 of the crystal was 19 millim. ; its thickness, taken in the direc- 

 tion of each of the two axes, 10 millim. The middle line be- 

 tween the two optic axes therefore corresponded to the shorter 

 diagonal of the rhombic prism. Upon two of its adjacent sides 

 two thin plates of tourmaline were cemented (which did not of 

 themselves exert any perceptible influence), so that polarized 

 light, which passed through both the optic axes of the crystal, 

 and then through the corresponding plate of tourmaline, yielded 

 one of the two systems of rings. The substance of the crystal 

 was diaraagnetic. 



1. When the crystal was suspended so that the plane of its 

 two optic axes could revolve vertically around its middle line, on 

 approximating the apices of the poles as much as possible, these 

 planes, in consequence of the diamagnetism, became placed 

 axially ; but, overcoming the diamagnetism, they rotated 90° and 

 became equatorial when the crystal was elevated or lowered. 



2. On suspending the crystal, so that the planes of its two 

 optic axes could oscillate horizontally, the middle line, in the 

 form of crystal above described, again assumed an axial position ; 

 and when elevated or lowered, overcoming the diamagnetism, 

 it became equatorial. 



33. In the experiments described in the last paragraph, the 

 topaz may be replaced by crystallized sugar. Thus, those planes 

 in the direction of which a crystal of sugar is most easily cleft, 

 are perpendicular to one of its two optic axes ; and w hen one of 

 these surfaces of transmission is polished (to effect which, mere 

 scraping with a piece of glass is sufficient) and placed between 

 two crossed plates of tourmaline, the plane of the two optic axes 

 is recognised even by the position of the black band seen through 

 the system of rings ; and we then know, from the known angle 

 of about 50° which these axes form with each other, the direc- 

 tion of the second axis and the middle line between them. 



34. Ciystals of Brazilian topaz, arragonite, nitre, sulphate of 

 soda, and of many other substances, which exhibited the pha^- 

 nomena of diamagnetism and crystallized in different prismatic 

 forms, when placed between the apices of the poles, as also at 

 every distance from them, arranged themselves with their longi- 

 tudinal and prismatic axis, which (although not in every instance, 



