70 



Magnetic Rotation of the Plane of Polarisation. [May 9, 



This value for Q/0 accounts for the fact that doubly refracting 

 bodies do not exhibit any rotation of the plane of polarisation,* if the 

 direction of the ray of light is inclined to the optic axis. When light 

 goes through a crystal in the direction of the optic axis, magnetic 

 rotation has been observed. E. Becquerel has observed rotation in 

 tourmalin and rock crystal, while Wertheimf has detected a well- 

 marked rotation in beryl, a feeble one in quartz, but nothing in 

 Iceland spar. 



[Liidtge J determined the magnetic rotation along the optic axis of 

 quartz and at various inclinations. The following table gives his 

 results and the corresponding values of /3 : — 



Inclination to optic axis 0° 1° 2° 3° 5° 



Rotation (Q) 1*1 1 '9 '6 '4 



Retardation (3) 13 52 117 325 



Liidtge adds that these figures are not to be taken as giving exact 

 measurements. ~No magnetic rotation has been observed in Iceland 

 spar even along the optic axis, and it is worthy of note that in this 

 crystal /3 very rapidly increases with the inclination to the optic axis. 

 If the length of the spar be 1 centimetre, then at an inclination of n° 

 to the optic axis for D line /3 = 318° X w 2 , while in quartz y3 = 17° X 

 n 2 . — May 14.] 



Wertheim has also shown that if a piece of heavy glass be com- 

 pressed, the magnetic rotation is diminished, even when the retarda- 

 tion due to the doubly refracting nature of the compressed glass is 

 much less than a wave-length. 



Ludtge's experiments on compressed glass show again how the 

 magnetic rotation is diminished as the doubly refracting property 

 increases. Liidtge gives the following table, where n represents part 

 of a wave-length retardation and d the magnetic rotation : — 



n 0-01 0-2 0-25 0-3 045 0'5 0-6 



<*.... 5° 4-6° 4-2° 4° 3-7° 3-5° 3° 2*4°. 



These results cannot be directly compared with what we should 

 expect from a crystal, since the ends of the glass are free from strain, 

 and a rotation is there produced, which is observed. 



Yillari's results are very similar to Liidtge's. Villari, by spinning 

 a disk of glass very rapidly, strained it, and on observing the magnetic 

 rotation found it get less and less as the strain got greater and 

 greater. There is, however, one noticeable difference between 



* Wiedemann, ' Die Lehre Ton der Elektricitat,' vol. 3, sec. 1097. 



f Wertheim, ' Compt. Rend.,' vol. 32. 



X Liidtge, ' Poggendorff, Annalen,' vol. 137. 



