The experiments of Obreimow & Schubnikoff (1927) on the birefringence produced by the plastic deformation of single crystals of rock salt have been extended to a polycrystalline material. Rolled sheets of silver chloride have been recrystallized and then deformed plastically in various ways-by simple extension and by bending, for example. The sheets are transparent and very ductile and, since silver chloride is cubic in structure, the birefringence patterns observed under the microscope provide a picture of the distribution of the internal stresses uncomplicated by natural double refractions. It is suggested that results obtained with this optical method are applicable to metals. Silver chloride appears to deform by glide, and when the glide packets are observed on edge a characteristic pattern of parallel birefringent bands is visible. The relation of the glide plane and glide direction to the crystal structure has been studied by making observations upon these bands and upon the glide lines formed on the surfaces of bars of square cross-section consisting effectively of chains of single crystals. The orientations of the fifteen sets of glide bands examined in this way were all consistent with glide movements in a <110> direction; the glide plane, however, was not always a crystallographic plane of low indices. In the six cases in which the measurement was possible, it lay within 9 degrees of the plane in the <110> zone on which the maximum shear stress, resolved in the <110> direction, acted. It is concluded that silver chloride deforms by 'pencil glide', the mechanism postulated by Taylor & Elam in 1926 to explain the plastic behaviour of $\alpha $-iron. The transmission of pencil glide across grain boundaries is discussed.