AbstractThe photoelastic birefringence of solids is caused by the superposition of two effects. An elastic deformation changes the arrangement of the atoms. If the deformation is a simple strain, this effect produces negative birefringence which increases with increasing refractive index of the solid. The second effect is due to a deformation of the atoms. A strain creates optical anisotropy of the atoms and causes positive birefringence. In most glasses the second effect is predominant, but for glasses with a large index of refraction the first effect takes precedence.