Mueller has developed an atomistic theory of photoelasticity for transparent, isotropic solids like glasses wherein the validity of Lorentz-Lorenz law is varifled. An extension of this theory and an empirical assumption that the fractional change in polarizability of an ion within the glass is proportional to the applied stress has allowed the present author to make a theoretical expression for the photo-elastic constants. This expression contains some atomic parameters which relate the change in polarizability to the applied stress. It is shown how these parameters can be calculated from an accurate experimental determination of the photo-elastic constants.