Quantitative measurements have been made of optical birefringence in two solid phases (α and β) of nitrogen. For the β phase (T > 35.6 °K), the observed birefringence is consistent with the structure determined by X-ray diffraction (P63/mmc) and it fixes the c/a ratio of the lattice accurately. While the measurements do not distinguish between the two structures (Pa3 and P213) that have been proposed for the α phase (T < 35.6 °K), they place a strict upper bound on the possible distortion that might arise from displacement of the nitrogen molecules from sites of cubic symmetry.