Max von Laue

“ Since Einstein's electrodynamics, [1] which is based on the principle of relativity, is equivalent with the older theory of Lorentz as long one restricts himself to the first power of the relations of all body velocities to the speed of light, it is obvious that it allows to calculate Fresnel's dragging coefficient as a first approximation. ”

“ In dispersive substances we have to fill in the value for n corresponding to the frequency in the primed system.
For the group velocity it is exactly the same, if we replace the refractive index n by the expression n + v (dn/dv) (v is frequency) .
So, according to the relativity principle, light is completely carried by the body, however, just because of this its velocity relative to an observer (who does not participate in the motion of the body) is not the same as the vector sum of its velocity relative to the body and that of the body relative to an observer. ”