It was recalled that it had been proposed that, when g•b = 0, the residual contrast of a dislocation in an isotropic crystal would always be stationary 2-sided. It was shown here that stationary 2-sided images occurred for g•b = 0 only when the crystal was sufficiently thick. In isotropic or anisotropic thin crystals, the dislocations could exhibit moving 1-sided images even though g•b = 0.

A.H.W.Ngan: Philosophical Magazine A, 1995, 71[6], 1209-18