The Magneto-optics of Iodine Vapour. 1009 



fundamental equations (1) given above to 



1 dE x j3 d 2 F x BE y _ldRy ft d*? y _ dE* R 



, D =0. 



and 1 dD* _ 3Hy 1 rfDy _ BH r 



c dt ~ ^z ' c dt ~ftz 



Under the conditions above specified these equations are 

 equivalent to the two relations 



(E,±;E y ) ( q *-i)=(P x ±iP l )(i+ i&y 



But we still have from equation (2) as above 

 , (E I± iE y )=( a± ^)(P x±? -p y ), 

 so that the equations for q are now 



-tf-D(i±^;-o, 



which, to the first order in /5, are the same as 



tf-i)(«±Mp)=o, ■ 



which are the same as in the text with 2/3 instead of /3. The 

 final formula for the rotation of the plane of polarization is 

 therefore exactly double that given in the text. G. H. L. 



CVII. The Magneto-optics of Iodine Vapour. 

 5t/R."W. Wood and Gr. Rebajjd *. 



THE magnetic rotatory polarization of iodine vapour was 

 discovered in 1906 by one of the present writers f. 

 A small glass bulb, highly exhausted and containing a 

 small crystal of iodine, was placed between the perforated 

 pole-pieces of a powerful electromagnet, and warmed until 

 the iodine vapour showed a light purple tint. Polarized 

 white light was now passed through the hollow cores of the 

 magnet and the bulb, and received by a Nicol's prism set for 

 the position of extinction. On exciting the magnet the nicol 

 transmitted light of a bright emerald-green colour, as a result 

 of the selective rotation of vapour due to the presence of 



* Communicated by the Authors. 



+ R. W. Wood, Phil. Mag. xii. p. 329 (1906). 



