 Choosing an appropriate angle of incidence (not yet exactly specified) gave a phase difference of 1/8 of a cycle.Two such reflections from the "parallel faces" of "two coupled prisms" gave a phase difference of 1/4 of a cycle.In that case, if the light was initially polarized at 45° to the plane of incidence and reflection, it appeared to be "completely" depolarized after the two reflections.