354 Mr. Yngve Bjornstahl on Magnetic 



angle of 45° with the lines of force. The telescope is 

 f ocussed on the sensitive strip, the angle between the principal 

 axis of the latter and that of the prisms being 45°. The 

 compensator is turned to get a field of view uniformly 

 illuminated, the magnetic field is applied, and the compensator 

 is set for a match again. 



From the rotation 6 of the compensator one obtains * for 

 the difference of phase 



tan A = 2 cos (20 o — 6) sin 6 sin 5, 



where 8 is the phase difference of the compensator and </> 

 is a constant. 



In order to analyse the elliptically polarized light emerging 

 from the tube G, it is sufficient to measure first the phase 

 difference between the components parallel and perpendicular 

 to the lines of force, then the azimuth of the major axis of 

 the ellipse, the elements of which are defined by this with 

 regard to form and situation. 



In order to determine the azimuth of the ellipse a A/4 

 plate L (fig. 1) was inserted between the tube G and the 

 compensator. The plate is rotated to get linear polarized 

 light, which is tested with the Brace apparatus. The azimuth 

 of the A/4 plate is read. This arrangement is indicated by 

 Cotton and Mouton f, who employed it for the study of 

 the dichroic circular polarization, and independently by 

 Bergholm J, who has given full particulars about the method. 

 Taking the liquid as a uniaxial dichroic crystal with the 

 axis along the lines of force, the absorption of the parallel 

 and of the perpendicular component is different. Calling 

 the rotation of the A/4 plate a/t and the specially determined 

 phase-difference A, we can derive the ratio p of the parallel 



amplitude to the perpendicular one p— — - from the equation 



2 -j 



— — = tan 2-\2r cos A. 



2p r 



The electromagnet is constructed by The Svedberg §. In 

 this investigation the distance of the poles was 3 cm. and 

 their diameter was 3 cm. The magnet is provided with a 

 cooling coil, through which water is circulated, and gives an 

 intensity of about 21,000 gauss, the current being 20 amperes. 



* Cf. Bergholm, he. cit. 



t Journ. de Phys. sor. 5, i. p. 9 (1911). 



X Loc. cit. 



§ Ami. der Phys. (4) lii. p. 657 (1917). 



