Birefringence in the Boundary Layer of a Rarefied Heat Conducting Gas

In a heat conducting diatomic gas enclosed between parallel plates, the second rank tensor polarization of the rotational angular momenta is non-zero only near the plates. Therefore the gas is birefringent only in a boundary layer which is a few mean free paths thick. The birefringence is calculated with the help of a differential equation and a boundary condition for the tensor polarization. The difference in the index of refraction for light polarized parallel, respectively perpendicular to the temperature gradient is evaluated for the gases N and CO with the information obtained from the field effects on viscosity and on thermal conductivity, and from thermomagnetic pressure difference measurements. The effect is estimated to be of measurable size.