1. A technique for inspecting a periodic structure for defects comprising the steps of: (a) placing the structure and at least one random dot stereogram in the field of view of a stereomicroscope having a pair of optical viewing paths; the at least one stereogram having a binocular disparity between left and right images thereof which is an integral multiple of the periodicity of the structure; (b) optically filtering each of the paths so that the left image of the at least one stereogram appears in one path and the right image appears in the other path; (c) simultaneous with step (b) viewing the structure and the at least one stereogram together; and (d) detecting the presence of flickering in the field of view of the stereomicroscope, said flickering being indicative of the presence and location of a defect.