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V. On the Constant of Magnetic Rotation of Light in Bisulphide of Carbon. 
By Lord Rayleigh, M.A., D.C.L., F.R.S. 
Received December 29, 1884—Read January 15, 1885. 
1. The phenomenon, to which the present investigation relates, is Faraday’s 
discovery of the “ Magnetisation of Light,” or in more usual language the rotation 
of the plane of polarisation of light in traversing certain media exposed to powerful 
magnetic force. One of the characteristics of this rotation is that it takes place 
in the same absolute direction which ever way the light may be travelling, differing 
in this respect from the rotation which occurs without the operation of magnetic 
force in quartz and many organic liquids. Advantage of this property has been 
taken by Faraday and others in order to magnify the effect. By reflecting the 
light backwards and forwards it is possible to make it traverse several times a field 
of force whose length is limited. 
A consequence remarkable from the theoretical point of view is the possibility 
of an arrangement in which the otherwise general optical law of reciprocity shall 
be violated. Consider, for example, a column of diamagnetic medium exposed to 
such a force that the rotation is 45°, and situated between two Nicols, whose 
principal planes are inclined to one another at 45°. Under these circumstances 
light passing one way is completely stopped by the second Nicol, but light passing 
the other way is completely transmitted. A source of light at one point A would 
thus be visible at a second point B, when a source at B would be invisible at A; 
a state of things at first sight inconsistent with the second law of thermodynamics. 
2. It is known that the rotation may be considered to be due to the propagation 
at slightly different velocities of the two circularly polarised components, into 
which plane polarised light may be resolved ; and it is interesting to consider 
what difference of velocity our instrumental appliances enable us to detect. A 
retardation amounting to one wave length (X), of one circularly polarised component 
relatively to the other would correspond to a rotation of the plane of polarisation 
through 180°. If we can observe a rotation of one minute, we are in a position 
to detect a retardation of X/10800. If l be the thickness traversed, v and v + Sr 
the two velocities of propagation, the relative retardation is l hv/v. To take an 
example, suppose that £=20 inches, X= 40 { )Q0 th inch; so that if Sv/v exceed 10“ 8 , 
the fact might be detected.* It appears therefore that we are able to observe 
extraordinarily minute relative differences in the velocities of propagation of the two 
circularly polarised rays. 
* Camb. Nat. Sci. Trip. Ex., 1883. 
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