Positive and Negative Optic Axes of Crystals. 451 



prism of tourmaline, staurolite, or " red ferridcyanide of po- 

 tassium," in my former experiments, will point neither axially 

 nor equatorially, but will take always a fixed intermediate 

 direction. This direction will continually change if the prism 

 be turned round its own axis B. It may be proved by a 

 simple geometrical construction, which shows that during one 

 revolution of the prism round its axis (B), this axis, without 

 passing out of two fixed limits C and D, will go through 

 all intermediate positions. The directions C and D, where 

 the crystal returns, make, either with the line joining the two 

 poles, or with the line perpendicular to it, on both sides of 

 these lines, angles equal to the angle included by A and B; the 

 first being the case if the crystal is a positive one, the last if a 

 negative one. Thence it follows, that if the crystal by any kind 

 of horizontal suspension should point to the poles of a magnet, 

 it is a positive one ; if it should point equatorially, it is a negative 

 one. This last reasoning conducted me at first to the law 

 mentioned above. 



The magnecrystallic axis, I think, is, optically speaking, 

 the line bisecting the (acute) angles made by the two optic 

 axes ; or in the case of one single axis, this axis itself. The 

 crystals of bismuth and arsenic are positive crystals ; antimony, 

 according to my experiments, is a negative one : all are uni- 

 axal. 



II. The cyanite is by far the most interesting crystal I 

 have examined. If suspended horizontally, it points very 

 well to the north, by the magnetic power of the earth only. 

 It is a true compass-needle, and more than that, you may 

 obtain its declination. If, for instance, you suspend it in 

 such a way that the line A bisecting the two optic axes of 

 the crystal be in the vertical plane passing through the axis 

 B of the prism, the crystal will point exactly as a compass- 

 needle does. By turning the crystal round the line B you 

 may make it point exactly to the north of the earth, &c. The 

 crystal does not point according to the magnetism of its sub- 

 stance, but only in obedience to the magnetic action upon its 

 optical axes. This is in full accordance with the different law of 

 diminution by distance of the pure magnetic and the opto-mag- 

 netic action. If you approach to the north end of the suspended 

 crystal the south pole of a permanent magnetic bar, strong 

 enough to overpower the magnetism of the earth, the axis 

 B of the prism will make with the axis of the bar (this bar 

 having any direction whatever in the horizontal plane) an angle 

 exactly the same it made before with the meridian plane, the 

 crystal being directed either more towards the east or more 

 towards the west, 



2G2 



