Let us call it -c squared, where c will be of the dimensions of a velocity.

This case yields the formulae of transformation which Larmor discovered for the transformation of Maxwell's equations of the electromagnetic field.

These formulae were extended by H.A.Lorentz, and used by Einstein and Minkowski as the basis of their novel theory of relativity.

I am not now speaking of Einstein's more recent theory of general relativity by which he deduces his modification of the law of gravitation.

If this be the case which applies to nature, then c must be a close approximation to the velocity of light _in vacuo_.