J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’

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                                Now we know that in each principal plane of a crystal 
the ray polarized in that plane obeys the ordinary law of  
refraction, and therefore its velocity is the same in whatever 
direction in that plane it is propagated. 

If polarized light consists of electromagnetic disturbances 
in which the electric displacement is in the plane of polarization 
then a<sup>2<\sup> = b<sup>2<\sup> = c<sup>2<\sup> (93) 
If on the contrary the electric displacements are perpendicular 
to the plane of polarization 
[lambda] = [mu] = [nu] (94) 

We know, from the magnetic experiments of Faraday, Plucker, 
&c that in many crystals [lambda] [mu] [nu] are unequal 
The experiments of Tyndall (Phil Mag 1852) on electric induction through crystals seem to show that 
a b & c may be different 

The inequality however of [lambda] [mu] [nu] is so small that great 
magnetic forces are required to indicate their difference, and 
the differences do not seem of sufficient magnitude to account for the double 
refraction of the crystals. 

On the other hand, experiments on electric induction are 
liable to error on account of minute flaws or portions of 
conducting matter in the crystal. 

Further experiments on the magnetic and dielectric properties 
of crystals are required before we can decide whether the 
relation of these bodies to <s>permanent<\s>magnetic and electric 
forces is the same when these forces are permanent 
or alternating with the rapidity of the vibrations of light. 
                            
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Manuscript details

Author
James Clerk Maxwell
Reference
PT/72/7
Series
PT
Date
1864
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7

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