Magneto-optic Properties of Crystals, 5 



in a more striking manner by this crystal. So strong is the 

 directive power, that a rhombus, suspended from one of its 

 obtuse angles, will set itself strongly equatorial, though its 

 length may be fifteen or twenty times its breadth. 



What is the conclusion to be drawn from these experi- 

 ments? We have first of all a diamagnetic crystal of pure 

 carbonate of lime, which sets its optical axis equatorial. On 

 substituting for a portion of the lime a quantity of protoxide 

 of iron sufficient to render the crystal weakly magnetic, we 

 find the position of the axis at once reversed. Replacing a 

 still further quantity of the diamagnetic lime by a magnetic 

 constituent, we find the action stronger, the force with which 

 the optical axis takes up the axial position increasing as the 

 magnetic constituents increase. These experiments appear 

 to be irreconcilable with the statement, that the position of the 

 optical axis is independent of the magnetism or diamagnetism 

 of the mass. 



Turning now to crystals possessing two optical axes, we 

 find the law of Plucker equally untenable ; a forcible contra- 

 diction is exhibited in the case of — 



Dichroite. — This crystal, as is well known, receives its name 

 from its ability to transmit light of different colours. The 

 specimen examined by us is a cube. In the direction of the 

 crystallographic axis, which coincides with the middle line, 

 the light transmitted is yellowish; through the other four 

 sides of the cube it is a deep blue. Suspended with the middle 

 line horizontal, whatever be the position of that line before 

 closing the circuit, the instant the magnetic force is developed 

 it turns with surprising energy into the axial position and 

 becomes fixed there. According to the law, however, the 

 middle line should stand equatorial, for the crystal is negative*. 



Sulphate of Barytes [heavy spar). — The form of this crystal 

 is a prism whose base is a rhombus, the four sides being per- 

 pendicular to the base. It cleaves parallel to the sides and 

 base. Suspended between the poles, with the axis of the 

 prism vertical, on exciting the magnet, the long diagonal sets 

 itself axial. The crystal is diamagnetic, and agrees, thus far, 

 with the carbonate of lime. Suspended from the acute angle 

 formed by two sides of the prism, the axis of the latter being 

 horizontal, on closing the circuit the said axis turns into the 

 axial position, and remains there as long as the force is pre- 

 sent. Suspending the crystal from its obtuse angle, the axis 

 being still horizontal, on closing the circuit the axis sets itself 

 equatorial. A plane perpendicular to the rhombic base, and 

 passing through the long diagonal, contains the two optical 

 * Brewster's list. 



