OF CRYSTALS BY THE POLES OF A MAGNET. 363 



appears to me that the empirical law laid down in paragraph 1, 

 so far as it relates to uniaxial crystals, is sufficiently established, 

 and applies indifferently both to positive and negative crystals. 



29. Hence it might further, with tolerable certainty, be sup- 

 posed, that an analogous action would occur also in crystals with 

 ttvo optic awes, to that found in uniaxial crystals. As such we 

 might expect either a repulsion of the tivo optic axes, or merely a 

 repulsion exei'ted against their central line, i, e. against that di- 

 rection which subdivides the acute angle formed by the optic axes. 

 Experiments are in favour of the first more universal assump- 

 tion, which comprises the latter. 



30. I cut a circular disc, about 22 millim. in diameter, from 

 a plate of mica, and suspended it by a silkworm thread so that 

 it could oscillate horizontally. As is well knoAvn, the two optic 

 axes of mica lie in a plane which is at right angles to the direc- 

 tion of the laminae in it, inasmuch as it forms with the normal 

 the same angle on both sides; this I estimated at 22\°. When 

 suspended as above, the planes of the two axes could rotate 

 around their mesial line placed vertically. Between the two 

 poles of the magnet, the lamina of mica assumed such a position 

 that its plane coincided with the equatorial plane. In this posi- 

 tion the equatorial direction was thus marked upon the lamina 

 of mica ; and it was afterwards found that the two optic axes, 

 i. e. those two directions which, when viewed by polarized light, 

 correspond to the central point of the two systems of rings, lie 

 in the same plane, which is placed at right angles to the plate of 

 mica in the equatorial direction. 



Mica exhibits the properties of a magnetic body. 



31. With the view of modifying the experiment described in 

 the last paragraph, with regard to the concluding remark, the 

 planes of the two optic axes of a plate of mica were determined, 

 and a hexagon with parallel opposite sides cut out of it, so that 

 its longest dimension, which was 26 millim., was in the plane 

 just determined, whilst the breadth of the lamina was only 18 

 millim. The lamina was then again suspended as before ; and 

 when the poles were approximated as much as possible, it ar- 

 ranged itself with its longitudinal direction, hence with the plane 

 of the two axes, in the line of the poles. When the lamina was 

 elevated or lowered, it rotated 90°, so that the planes of the 

 two optic axes became perpendicular to this line. 



32. I then took a transparent and colourless topaz from Scot- 



