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

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                                (100) The equations of the electromagnetic field, deduced from purely 
experimental evidence, show that transversal vibrations only 
can be propagated. If we were to go beyond our experimental 
knowledge and to assign a definite density to a substance which 
we should call the elastic fluid and select either <s>positive<\s> vitreous or <s>negative<\s> resinous 
electricity as the representative of that fluid, then we might 
have normal vibrations propagated with a velocity depending on 
this density. We have however no evidence as to the density of electricity 
as we do not know whether to consider vitreous electricity as 
a substance or the absence of a substance. 

Hence electromagnetic science leads to exactly the same 
conclusions as optical science with respect to the <s>nature<\s> direction 
of the disturbances which can be propagated through the field, 
<s>each<\s> both affirm the propagation of transverse vibrations and both 
[indicate/present?] the same velocity of propagation. On the other hand 
both sciences are at a loss when called on to affirm or deny 
the existence of normal vibrations. 

(101) Relation between the Index of Refraction and the electromagnetic 
character of the substance 

The velocity of light in a medium according to the Undulating 
Theory is 1/i V<sub>0<\sub> 
where i is the index of refraction and V<sub>0<\sub> is the velocity in vacuum 
The velocity according to the electromagnetic theory is 
[square root]k/4[pi][mu] 
where k = 1/D k<sub>0<\sub> and k<sub>0 = 4[pi]V<sub>0<\sub><sup>2<\sup> 
Hence D = i<sup>2<\sup>/[mu] (80) 
or the <s>refr Index<\s> Specific Inductive Capacity is equal to the 
square of the index of refraction divided by the coefficient of 
magnetic induction 
                            
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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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