A comparison of the elastic and the electrical theories of light with respect to the law of double refraction and the dispersion of colors

J. W. Gibbs
1888 American Journal of Science  
This will appear most distinctly if we consider that V divided by the velocity of light in vacuo gives the reciprocal of the index of refraction, and p multiplied by the same quantity gives the wave-length in vacuo.
doi:10.2475/ajs.s3-35.210.467 fatcat:onzfn3tkdffj5e6jjaiccxoiay