Magneto-optic Phenomenon. 265 



Unfortunately quantitative observations with the poorly re- 

 flecting magnetite could not be pursued beyond blue. Further 

 measurements might have proved the more interesting, as the 

 rotation appeared almost to vanish for violet. I do not deem 

 it impossible that the rotation passes through zero and changes 

 sign in the ultra-violet, though this point appears difficult to 

 decide. 



Third P^kt. 



§ 23. The combined expression of both experimental laws, 

 e = K3 and e=e cos (3, 9?), leads to the general law : — 



e=K3cos(3,^) = K3„, 



where ^ n stands for the component magnetization normal to 

 the mirror. This relation was found for red light, but may 

 evidently be extended to radiations of all wave-lengths ; it 

 may be thus expressed : — 



V. The rotation of the major axis of vibration of radiations 

 normally reflected from a magnet is algebraically equal to the 

 normal component magnetization, multiplied into a constant K. 



Poisson, as is well known, has shown how any arbitrary 

 magnetic distribution may be replaced, without prejudice to 

 its internal or external action, by fictitious magnetic fluids, 

 distributed in a definite manner throughout the magnet and 

 on its surface*. The surface-density 3 of the latter portion 

 is given by the equation $ = j n - By the above we now 

 have the rotation proportional to the surface-density, and it 

 would thus appear theoretically possible to determine the 

 latter by purely optical means. It would, however, be of no 

 practical use to introduce the fictitious quantity & into the 

 physical equation ; it has no meaning beyond that of a purely 

 mathematical symbol. 



§ 24. The constant K is a quantity of dimension [U M~*T] 

 in electromagnetic measure. For four substances it is given 

 in Table II. as a function of the wave-length ; it hardly varies 

 with temperature. I venture to propose calling it "Kerr's 

 constant;" for another quantity, which I have previously 

 defined and denoted M^f, the name " Kundt's constant" 



* Not to be confounded with Gauss's surface distribution, the action of 

 which may be substituted for that of the magnet only for points external 

 to it. Both distributions do not coincide except in the particular case of 

 solenoidal magnetization. 



t The constant ^ for radiations of definite wave-length is algebraically 

 equal to the rotation which the plane of polarization of such radiations 

 would experience on normal transmission through a plate of unit thick- 

 ness and unit normal magnetization ; du Bois, Wied. Ann. xxxi. p. 963 

 (1887). 



Phil. Mag. S. 5. Vol. 29. No. 178. March 1890. X 



