1889.] Rotation of the Plane of Polarisation of Light. 



3 



elusions of the theory, especially with regard to zero and negative 

 rotation. It concludes with the words : 



"Ebenso darf man bei Krystallen, an denen man keine elektro- 

 magnetische Drehung beobachten kann, nicht den Schluss ziehen, 

 dass ihnen keine Verdet'sche Constante zukame, weil eine vorhandene 

 Drehung durch die superponirte Doppelbrechung geschwacht wird 

 und bis zur Unmerklichkeit verdeckt werden kann." 



In the second part he commuDicates the repetition of Yillari's 

 experiment on the rotation of the plane of polarisation of light in a 

 disk spinning between magnet poles. He finds a diminution of rota- 

 tion from 5'06° to 0*77° when the disk spins 10,800 times in a 

 minute, and proves experimentally that this is due to the double 

 refraction produced by centrifugal force. For, on the one hand, the 

 double refraction produces a difference of phase in the linear com- 

 ponents, which, like the centrifugal force, is proportional to the 

 square of the number of revolutions, and hence must be due to this 

 force ; on the other hand, the rotation vanishes as the theory 

 requires, just where the difference of phase of the linear components 

 is sr. 



Thus Mr. Ward's essential results, namely, the diminution of the 

 rotation by double refraction, and the explanation of Yillari's experi- 

 ment by means of the double refraction due to centrifugal force, have 

 been already communicated in the above-named papers. 



With regard to Mr. Ward's mathematical deductions, the first of 

 the before-mentioned authors wishes to point out a mistake which 

 Mr. Ward has made in forming his differential equation. 



In accordance with Mr. Ward, we let the x or y axis fall in the 

 plane of principal section, and the z axis in the direction of the rays 

 of light. We must examine how an ellipse is altered under the 

 common action of rotation and double refraction, if we advance in 

 the medium a short distance dz. Like Mr. Ward, we take the ellipse 

 as produced by composition of y and x components, the ratio of whose 

 amplitudes is tan a, and whose difference of phase is y3 ; then the 

 direction of the major axis forms with the x axis an angle o>, deter- 

 mined by the equation 



tan 2(v = tan 2a cos /? (1.) 



Since the double refraction does not alter the value of a, we obtain 

 the variation of w with /3 under the influence of double refraction 

 alone by forming do/dp, regarding a as constant — 



— = — J cos 2 2w tan 2a sin p, 

 dp 



or ' ~ = -\ sin 4w tan j3. (2.) 



dp 



b 2 



