Refractive index measurements of uniaxial media (crystals and a polymer liquid crystal in the nematic phase) are described. The technique used, applicable for any anisotropic uniaxial medium, makes use of the birefringence to generate interference between the extraordinary and ordinary waves created after an incident plane polarized wave enters the crystal. The dispersion of the refractive indices for the uniaxial nematic polymer is discussed together with the implication it has on the third order nonlinear optic susceptibility measurements completed using the Third Harmonic Generation (THG) technique.