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Y. On the Theory of the Magneto-Optic Phenomena of Iron, Nickel, and Cobalt. 
By J. G. Leathem, B.A., Fellow of St. Johns College, and Isaac Newtoyi Student 
in the University of Cambridge. 
Communicated by Sir Robert S, Ball, F.B.S. 
Received May 11,—Read June 17, 1897. 
Introduction. 
1. In his ‘British Association Report’ (1893), on the “Action of Magnetism on 
Bight, Ml. B ARMOR points out that there are two possible ways in which the 
magnetic field may be regarded as affecting the phenomena of light propagation. 
The imposed magnetisation is an independent kinetic system of a vortical character, 
which is linked on to the vibrational system which transmits the light waves,” and 
from the first point of view “ the kinetic reaction between the two systems will add 
on new terms to the electric force,” and so there would be a “magneto-optic term” 
in the expiession for the kinetic energy. This type of theory includes Maxwell’s 
hypothesis of molecular vortices, and has been analytically treated by FitzGerald 
ana Basset ; it also includes the theory developed by Crude in his paper “Magneto- 
optische Erscheinungen,” in ‘ Wiedemann’s Annalen,’ vol. 46. The great difliculty'’arises 
when one comes to consider the boundary conditions, as a discontinuity of electric force 
cannot be avoided ; apparently the only satisfactory way of meeting this difficulty 
is to be found in Barmor’s suggested modification of FitzGerald’s analysis, involving 
the supposition that in the case of reflection of light at the surface of a magnetised 
metal the constraint introduces an irrotational or compressional wave of the ether set 
up at the reflecting surface and travelling with very great or infinite velocity through 
the space occupied by the metal; a satisfactory system of equations of propagation 
and boundary conditions is thus obtained by applying the principle of Beast Action. 
I have worked out the mathematics of this theory and obtained the general solution 
of the problem of reflection from a magnet; on comparing this with the experimental 
results of several German and Dutch physicists, it appears that the agreement of the 
theory with^ experiment is at best very doubtful, even when allowance is made for 
the possibility of large errors of observation. There is moreover one phenomenon, 
recently discovered, which the theory quite fails to account for, viz., an effect of the 
component of the magnetic field perpendicular to the plane of incidence. 
2. The second type of theory supposes that the imposed magneti.sation “slightly 
