The use of silver chloride as a material for ``photoelastic'' stress analysis offers the possibilities of studying both elastic and plastic states of stress in a crystalline material on either a micro‐ or macroscale. It is necessary, however, to relate quantitatively the stress state and the observed relative retardation and extinction angle. In this paper these relationships are developed from a general theory of stress birefringence, according to a stress‐dependent hypothesis. This hypothesis and the resulting analytical relationships are experimentally vindicated by measurements made on a variety of single crystal specimens of silver chloride tested in simple tension in the elastic and plastic stress ranges.