472 Prof. D. B. Brace on the Resolution of Light into 



light entering the combination at right angles to the lines of 

 force. According to Verdet's law the Faraday " effect " in 

 this direction is zero. In experiments'* I have found this 

 to hold down to an angle of O'OSS' with the normal to the 

 lines of force. I have furthermore shown that when the 

 Faraday " effect " is eliminated there is no double refraction, 

 at least the relative retardation must be less than 2*8 x 10 " 7 

 per centimetre in a field of 2500 C.G.s. units for &)d = , 115 / . 

 Assuming the incident ray a to be unaffected, the reflected 

 ray a will be broken up into two opposite circular components, 

 making an angle o£ *52" with each other, and equal angles 

 with the original ray. The next total reflexion produces a 

 relative change in phase of each of the linear components of 

 the circular vibration, changing them into elliptical vibrations. 

 The next total reflexion will produce a like amount. As the 

 direction of rotation of the plane in the Faraday " effect" is 

 independent of the direction of propagation, but only depends 

 on the absolute direction of rotation of the Amperian currents, 

 in order that the field may impress a like change on the ray 

 after the third reflexion it must be circular and have the 

 same direction of vibration relative to Amperian currents. 

 This requires that the ray shall have a total change of phase 

 of \\ after the third reflexion, to give it the same absolute 

 direction of vibration in space. To effect this a mica plate, 

 with its principal axes placed respectively parallel and at 

 right angles to the lines of force, was carefully split and tested 

 with two total reflexions until the entire retardation was 

 exactly -JA D . This must be accurate as, if there is a slight 

 ellipticity in the ray, this residual element remaining after 

 the subtraction of the circular component will distribute light 

 between the two circular elements and destroy definition 

 when examined in the telescope. After the third reflexion 

 (or the final reflexion), the two components will be separated 

 and form two images in the telescope, that one which has the 

 greater velocity or acceleration in the magnetic field forming 

 an image on the right, and that which has the less on the left. 

 In the first experiment, a double slit was used whose ele- 

 ments were each 06 millim. wide and 0*5 millim. apart and 

 distant 10 metres from the prisms, the angle thus subtended 

 being '0005 absolute unit. On account of the great absorp- 

 tion and loss of light from so many internal passages, an 

 intense source was necessary. All the usual methods failed, 

 but by fusing sticks of common salt and placing them just 

 at the bottom edge of a crucible broken in half and directing a 

 powerful oxyhydrogen jet on the edge of the same, sufficient of 

 * Phil. Mag. [o] xliv. p. 342 (1897). 



