The refraction per gram atom of oxygen, defined by a formula of either the Gladstone-Dale, the Newton, or the Lorentz-Lorenz type, equals the sum of terms characteristic of the component "metallic" elements, each of these terms being proportional to the number of atoms of that element per atom of oxygen. This is shown to be true (with but little error) for practically all classes of silicate glasses for which adequate data are available for testing the relationship. (Deviations from strict proportionality are, however, observed for glasses containing large amounts of lead.) Constants are derived empirically for use in the Gladstone-Dale and Newton type equations. Different constants are needed for triangularly surrounded and for tetrahedrally surrounded boron atoms. Using the equations previously derived for the calculation of the density and volume per oxygen, it is now possible to compute the refractive index (for the sodium D lines) of a wide variety of silicate glasses from the composition alone.
MAURICE L. HUGGINS, "The Refractive Index of Silicate Glasses as a Function of Composition," J. Opt. Soc. Am. 30, 495-504 (1940)