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Published online by Cambridge University Press: 24 October 2008
The two letters now communicated are from G. G. Stokes to W. Thomson, of dates Dec. 12–13, 1848, three years after Faraday's great magneto-optic discovery. They formulated already the permissible types for general equations of propagation, virtually on the basis of the very modern criterion of covariance,—relative to all changes of the spatial frame of reference in the case of active fluids, but having regard to the fixed direction of the extraneous magnetic field in the Faraday case. Their form was elucidated in each case by correlation with a remarkable and significant type of rotational stress in a propagating medium.
* Lord Kelvin graduated in the Mathematical Tripos of 1845, and soon after became Professor at Glasgow: Sir George Stokes became Lucasian Professor in 1849.
* These are not covariant: they are corrected in the letter following.
* The signs of (ω′, ω′′, ω′′′) here should be changed.
* For they are correlative to the stress in an isotropic incompressible elastic solid for which (θx, θy, θz) is the displacement.
