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



in its intimate connexion with magnetization. Optical com- 

 plications were purposely avoided by working at almost normal 

 incidence. I have also determined the effect of temperature, 

 the rotational dispersion, and the characteristic absolute 

 constants. 



§ 2. Method. — Light from a Linnemann's burner (incan- 

 descent zirconia disk) was made to pass horizontally in suc- 

 cession through red glass, a lens, a Lippich's penumbral 

 polarizer ; it then reached the vertical mirror of the magnet y 

 by which it was reflected into the analyser and telescope. By 

 special constructive devices the parts of the apparatus, passed 

 through by the light before and after reflexion respectively, 

 could be fixed on a common base so near to each other 

 as to have their optical axes inclined at but 2°; this angle is 

 halved by what I shall call the apparatus's medial line. The 

 observer's eye was, of course, carefully screened from the 

 burner. The medial line was adjusted normal to the mirror. 

 By this arrangement practically normal incidence (at 1°) was 

 ensured ; in fact, Bighi * noticed hardly any change in the 

 phenomenon up to 15° of incidence. 



The same physicist has found that with plane-polarized light, 

 normally incident, the reflected vibrations are exceedingly 

 oblong ellipses, the azimuth of whose major axis naturally 

 differs from that of incidence. I saw no reason for making 

 measurements of ellipticity ; working with a penumbral 

 polarizer, the azimuth of the major axis is what is actually 

 observed. When using good mirrors and avoiding all dif- 

 fused light, the alternative extinction of each half of the field 

 of vision proved satisfactory, thus showing the ellipticity to 

 be very small. The analyser's azimuth was measured by a 

 modification of PoggendorfFs mirror-and-(vertical)-scale 

 method f. The -arrangement and dimensions of lenses, dia- 

 phragms, the telescope, &c, had been calculated beforehand 

 with a view to obtain the field of vision as bright and uniform 

 as possible, 



§ 3. The metallic cores principally used were ovoids (pro- 

 late ellipsoids of revolution) of different material, on to which 

 small reflecting-planes had been ground in various positions. 



* Righi, Ann. Chim. et Phys> [6 s&.] ix. pp. 120 and 132 (1886). 



t [This was separately described (Wied. Ann. xxxviii. p. 494, 1889) in 

 order not to encumber the present paper with details, and does not pre- 

 tend to anything beyond a mere laboratory expedient. It has since come 

 to my notice that other experimenters have used similar devices.] 



