120 
ME. .1. G. LEATHEiM ON THE THEOEY OF THE 
nuio-netisation was a little over 1100 C.G.S., the rairror being of iron. Under these 
circumstances he found the acceleration of phase to be '003 X 90° with a mean error 
of -001 X 90°. 
Calculating the theoretical value, we notice that 
Y = *2249, y—— (62° 43'), 
Y'= -4601, y = 25° 44', 
and if we assume x — 63° (which i.s a.bout the mean of the values indicated by the 
experiments of Sissingh and Zeeman), then 
.a; + 90° -y- y' — 4a = 360° + 44° 39'; 
we may also assume 
logio ( - ^o) = n-8623. 
With these values we find that the change of phase indicated by the theory is, in 
circular measure, 
•003818, 
or, in degrees, 
•00243 X 90°. 
This agrees very well with Zeeman’s observations. 
Transmission throngh Metal Films. 
30. Another effect of the action of magnetism on light is the rotation of the plane 
of polarisation of normally incident light, on passing through very thin films of 
magnetised metal. The principal experiments in this subject have been made by 
KUxXDT, Du Bots, Lobach, and Deude ; they found that the rotation is always in 
the direofion of the magnetising current, and measured it in special cases. It is 
desirable to compare these measurements with the mathematical solution of the 
problem worked out on the basis of the present theory. 
Let the film be bounded by the planes 
2=0 and 2 = — h, 
and let the incident lio’ht fall normally on the surface z = 0. The external 
magnetic field is supposed to be parallel to the axis of s, so that = 0 and (3^ = 0. 
The plane of polarisation of the incident light is taken as the plane of yz. 
Thus we may assume the following expressions to represent the light in the air on 
the two sides of the film, and in the film itself. 
