118 
MR. J. G. LEATHEM ON THE THEORY OF THE 
tor steady ones. There is nothing nnnatiiral in this, for the incipient conductions 
whicli make optical opacity have no relation of continuity whatever with the steady 
conduction in an ordinary current; thus Maxwell found that the ordinaiy coeffi¬ 
cients of “ conductivity ” are very much smaller in the optical circumstances. And 
it may be noticed that, as € is proportional to electromotive force divided by current, 
a greatly diminished conductivity will correspond to a greatly increased value of §. 
The value which Hall’s constant would, on this supposition, have for yellow light, 
is obtainable from the equation 
[maginary part of = imaginary part of — S’ 
wherein I now take the Hall effect to be proportional, not to the magnetic force, 
but to the intensity of magnetisation. 
This gives 
^ = 
4cA- 
R^ sin 4a 
C,^ sin X 
= + (5’6 70) for iron, and yellow light. 
The real part of p.j is then Cq sin x cot 4a. /3q, so that, if 6^, b^) — Eq (ag, yg). 
and hence we find 
~ = C(j cos X — Cq sin x cot 4a 
sin {x — 4a) 
sin 4 a 
logioEu = 41-0939, Eo = (1-242). lO'^k 
Effect of Magnetisation Perpendicular to the Plane oj Incidence. 
29. A very interesting inference from the presence of rj^ in the equations (36) is 
that, if the present theory be true, the component of magnetisation perpendicular to 
the plane of incidence will produce an eftect not quite the same as the Kerr pheno¬ 
menon, but of the same order of magnitude. 
On enquiring wdrether such an effect had ever been observed or measured, I 
found that a few months ago it wms predicted from theoretical considerations Dy 
Dr. C. H. Wind, in a paper which has as yet appeared only in Dutch. Acting on 
this prediction Zeeman sought the phenomenon experimentally, found it, and 
succeeded in measuring it. His results are published in the ‘ Commuiiications from 
the Leiden Laboratory of Physics,’ No. 29. 
Let us suppose that the magnetisation is entirely perpendicular to the plane of 
incidence ; then = t) and 173 = 0 , and the reflected light is specified by A and B, 
where, from equations (36), 
