126 
MR. J. G. LEATHEM ON THE THEORY OF THE 
To compare this with the rotation indicated by theory, we notice that 
hj\= *1391, and 27rhj\= '8740. 
So that 
F/E= - (2-805) + t (2*063) + (-7500) 
(the other term being in this case so small that it may be neglected) 
= - t.c\Co7o6‘"Tl~’“e-"‘“ (3-934) 
= oxc„y„?||ca— 
-^•6-^4 
_ pXt! V —— ,^‘(107° 41') 
- ;l5.87 ® 
X being taken as 63° 
The angle 0 is the real part of this complex, and is therefore -01677 in circular 
measure, being positive when yy is positive, so that it is a rotation in the ihrection ol 
the magnetising current. I’his theoretical value of 0 in degrees is -961. As we have 
seen, the observed value in degrees is 1-62 ; in comparing these results it should be 
noticed that one factor of the theoretical value of 0 is the cosine of an angle which 
is just about 17° greater than a right angle ; this angle contains x, whose value we 
have had to guess ; a comparatively small error in the value assigned to x will there¬ 
fore make a considerable error in the calculated value of 0. The values of Cg and yg 
being also uncertain, the agreement of the theory with experiment may be regarded 
as good. 
In one of Drude’s experiments h/X = -332, and the light used is red ; the observed 
rotation is 4*25 degrees. If we substitute this value of h/X in the above-obtained 
approximate formula, we find 
F/E = - t.cXCgyge‘^E-A--‘“ (8-715) 
S;7_15 
M 7 o ;^5.8y 
cXQy. 
whence 0 — -05187 in circular measure, or 2-972 degrees. 
In this case, in addition to the possible causes of error referred to in connexion 
with the previous experiment, it is to be noticed that though the experiment was 
made with red light, it has been necessary in the calculation to use the values of Cg, 
X, E, and a for yellow light, for lack of information as to their values in the case of 
red light. When also it is borne in mind that the value of the magneto-optic 
constant derived from observation of reflection from mirrors has here been applied to 
test experiments on transmission through thin films, with results not only of the 
same order of magnitude but identical within the limits of uncertainty of the 
