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



the magnetizing field. The rotation e proved very nearly 

 proportional to JT ; for each ovoid the mean m of the ratios 

 € o/3' was therefore calculated. By marking the values of 

 € /nt in the figure, and joining these points ©, lines are 

 obtained consisting of straight parts nearly coinciding with 

 the broken curves, so as even partly to overlap in the small 

 diagram. Proportionality is now seen to exist between € 

 and 3 ; at the first glance. The optical phenomenon, however, 

 can evidently only depend upon the magnetic condition imme- 

 diately behind the mirror ; the following conclusion therefore 

 appears justified : Within the comparatively narrow range of 

 fields applied (.0 between 100 and 900 C.G.S.), the distribu- 

 tion remains unchanged ; accordingly, 3, the magnetization 

 at the mirror, is proportional to 3'? the mean magnetization 

 throughout the ovoid. 



I. This gives the Law of Proportionality : 



6 =K3, 



where K is a constant. 



In order to give this result a solid experimental foundation 

 I have worked with 5 ovoids of different metals and various axial 

 ratios. Accordingly each curve is seen to exhibit its peculiar 

 character between the extremes of steel and nickel, though 

 in each case verifying the proportionality to be proved. 



§ 10. Inclined temporary magnetization. — All researches 

 hitherto published refer either to "polar" (a = 0) or to 

 "equatorial " (a = 90°) reflexion. The case of magnetization 

 inclined to the mirror's normal remains to be investigated. 

 On to a large ovoid (2a =15 ; 26 = 3 ; n=l/5) of soft 

 Swedish iron small elliptical mirrors (minor axes *25 centim.) 

 were ground under different angles to the axis of revolution. 

 These were so small in comparison with the ovoid that the 

 latter's longitudinal magnetization might be considered uni- 

 form. Accordingly the rotation observed may be estimated 

 (by data of § 19) at 90 to 95 per cent, of that with an in- 

 finitely small mirror. The inclinations of the mirrors to the 

 axis of revolution were measured in the lathe ; the angles a. 

 are their complements. 



§ 11. The rotation was now measured for 4 mirrors with 

 the same current flowing in the coil, and the beam of light 

 passing forwards and backwards through the looking-tubes 

 mentioned in § 3. The value zero for equatorial reflexion at 

 normal incidence is taken from previous observers. 



The numbers in the last line were calculated by analogy to 

 Yerdet's cosine law. The apparatus for these last experi- 



