Effect of the internal field on the birefringence of polymer crystals

The discrepancy between the values of the anisotropy of methylene groups determined from crystal refractive indices, stress‐optical data, and gas and liquid light‐scattering depolarization is explained on the basis of the effect of the internal field on the polarizability of the isolated molecule. The internal field may arise from intermolecular or intramolecular interactions which depend upon molecular conformation and state of aggregation. A simple continuum calculation based upon an extension of the calculation of the Lorenz‐Lorentz field is shown capable of accounting for the discrepancy.