262 H. E. J. G. du Bois on Kerr's 



and index of refraction*. It may also be remarked that Dr. 

 Sissingh f has not been able to observe any change in the two 

 ordinary constants of reflexion for iron between 20 c and 120°. 



Second Part. 



§ 16. The experiments in which the electromagnet was 

 used remain to be described. Polarizer and analyser now 

 consisted of genuine Lippich prisms %. Working with a 

 parallel beam of light was now given up, and an image of 

 the zirconia disk thrown on the analyser's diaphragm instead ; 

 by this artifice of Lippich's the disturbing effect of any 

 flickering of the source of light on the accuracy of observa- 

 tion is counteracted. Distance of vertical scale is 400 centim. 

 nearly. These improved arrangements, together with the 

 mounting of the optical apparatus on a pillar, separate from 

 the electromagnet, considerably increase the accuracy of the 

 results stated below above that hitherto attained. 



§ 17. Other active substances. — Besides the three metals, 

 exhibiting negative rotation on reflexion, I have detected 

 Kerr's effect on common magnetic iron-ore. A piece of 

 loadstone, ground and polished, showed a slight rotation be- 

 tween the poles of an electromagnet. Its numerical value 

 varies over the mirror, but it is always positive. For quanti- 

 tative measurement this material is of no use on account of its 

 heterogeneous structure. 



I therefore got a mirror ground parallel to the octaheder 

 (11 l)of a small crystal of magnetite (Fe 3 0. 4 , holohedr. tesseral). 

 With a strong current in the electromagnet this gave a 

 rotation e =+5 / for red light; the extinction was perfect, 

 proving the ellipticity to be quite negligible. The rotation 

 proved independent of the azimuth of incidence relatively to 

 the crystal's principal directions ; I therefore abstained from 

 having other mirrors ground on in different positions, mag- 

 netite evidently behaving like an isotropic body. It would 

 be interesting to observe the effect on transmitted light. 

 However, hitherto I have not been able to find any method of 

 procuring transparent films of this highly opaque substance. 



§ 18. I have further tried the following much less magneti- 



* Kundt, Berl. Berichte, Dec. 1888, p. 1393. 



t Sissingh, Dissert., Leiden, 1885, p. 136 ; Arch. Neerl. xx. p. 216 

 (1886). 



X Lippich, Wien. Ber. xci. 2, p. 1081 (1885). These prisms, as made 

 by Dr. Steeg and Reuter, of Homburg, give a perfectly uniform field of 

 vision with a well-defined line of demarcation ; unfortunately, the linseed- 

 oil used in their construction requires months for drying, so that I could 

 not use them from the beginning. 



