334 MAGNETISM 



of the plane of polarisation. Further, it changes sign with N n ; 

 or when n < N,z^ >P n , and when n is > N, v p < *> The theory 

 of which this is a mere indication led Voigt to the result just 

 obtained, and it was verified by Voigt and Weichert in the case of 

 sodium vapour.* 



Magnetic double refraction in colloids due to sus- 

 pended particles. In 1901f Kerr observed that if a small 

 quantity of Fe 8 O 4 is chemically precipitated in invisibly fine 

 particles in water, which is thereby rendered slightly hazy, the 

 liquid becomes doubly refracting when it is placed in a magnetic 

 field and is traversed by a polarised ray perpendicularly to the field. 

 The vibrations in which the electric component is along the lines of 

 force, travel more rapidly than the vibrations in which it is at 

 right angles. At the same time the former are more absorbed. 

 This agrees with the supposition that the particles of the oxide 

 arrange themselves in fine filaments along the lines of force, and 

 act like a Hertz's grating of parallel wires. 



Very shortly after this, MajoranaJ independently discovered the 

 phenomenon in colloidal solutions of iron, and found that it \va^ 

 very notable in long-prepared "Fer BravaU." Cotton and 

 Mouton also worked at the subject, examining many colloidal 

 solutions. 



Magnetic double refraction in pure liquids. At first 

 Cotton and Mouton supposed that the colloidal condition was 

 necessary for magnetic double refraction, but they discovered that 

 it is exhibited by nitrobenzene, and it has been observed in other 

 aromatic compounds. The phenomenon is c-losely parallel to the 

 Kerr electric effect. If 8 is the retardation of one ray behind 

 the other in traversing length /of a substance placed in >i field H 

 which is at right angles to the path, and if X is the wave length 



C is the constant of magnetic double refraction for the substance 

 for the wave length used. Cotton and Mouton found that for nitro- 

 benzene C increases as X diminishes, and that if B is the Ken- 

 electric constant C/B is very nearly the same for different wave 

 lengths. This has been confirmed for other aromatic compounds 

 by McComb and Skinner.|| 



A theory of magnetic double refraction has been developed by 

 Havelock,H who supposes that the molecules are differently spaced 

 along and perpendicular to the field. Cotton and Mouton prefer 



* Wiffd. Ann. 67, p. 345 (1899) ; or Wood's Optic*, chap. xvii. 

 t Brit. Asuoc. Report (1901), p. 568. 

 i Science Abstracts, 1902, No. 1844. 



Ann. dr. C/iim. ct de Phys. [8], vol. xi. p. 146 and p. 289 for their earlier 

 work; Journal de Physique [6], vol. i. (1911), p. 5, for their later discovery. 

 j| Phys. Rev. xxix. p. 525 and p. 541 (1909). 

 f Prop. Roy. /Sue., A, vol. Ixxvii. p 170, and vol. Ixxx. p. 28. 



