Magneto-optic Properties of Crystals. 29 



the magnetic attraction, necessarily triumphed when the poles 

 were removed to a sufficient distance. The same crystal, 

 however, between a pair of flat poles, could never take up the 

 axial position. On bringing the faces within half an inch of 

 each other, and exciting the magnet by a battery of thirty- 

 two cells, the crystal vibrated between the faces without 

 touching either. The same occurred when one cell, six cells, 

 twelve cells, and twenty ceils, respectively, were employed. 



If the attraction increases, as stated, more quickly than the 

 hypothetic repulsion, how can the impotence of attraction in 

 the case before us be accounted for? We have here a power- 

 ful current, and poles only half an inch apart; power and 

 proximity work together, but their united influence is insuffi- 

 cient to pull the crystal into the axial line. The cause of the 

 phaenomena must it seems be sought, not in optic repulsion, 

 but in the manner in which the magnetic force is applied. 

 The crystal is strongly magnetic, and the pointed poles exer- 

 cise a concentrated local action. The mass of both ends of 

 the crystal, when in the neighbourhood of the points, is power- 

 fully attracted, while the action on the central parts, on ac- 

 count of their greater distance, is comparatively weak. Be- 

 tween the flat poles the crystal finds itself, as it were, totally 

 immersed in the magnetic influence; its entire mass is equally 

 affected, and the whole of its directive power developed. The 

 similarity of action between the flat poles and the points, with' 

 drawn to a distance, is evident. In the latter case, the force, 

 radiating from the points, has time to diffuse itself, and fastens 

 almost uniformly upon the entire mass of the crystal, thus 

 calling forth, as in the former case, its directive energy; and 

 the equatorial position is the consequence. The disposition 

 of the lines of force, in the case of points, is readily observed 

 by means of iron filings strewed on paper and brought over 

 the poles. When the latter are near each other, on exciting 

 the magnet the filings are gathered in and stretch in a rigid 

 line from point to point; according as the poles are with- 

 drawn, the magnetic curves take a wider range, and at length 

 attain a breadth sufficient to encompass the entire mass of the 

 crystal*. 



As the local attraction of the mass in the case of magnetic 

 crystals deranges the directive power and overcomes it, so 

 will the local repulsion of the mass in diamagnetic crystals. 

 A prism of heavy spar, whose length was twice its breadth, 

 hung from its acute angle, stood between the flat poles axial, 

 between the points equatorial. On making its length and 



* Mr. Faraday has already pointed out " the great value of a magnetic 

 field of uniform force."— Phil. Trans., 1849, p. 4. 



