(1) First one and then the other of a pair of corresponding images (photographic or other) are projected alternately and in rapid succession onto a flat surface such as a table or screen, or into the optical system of a photogrammetric apparatus for simultaneous viewing with both eyes. The viewer sees a flickering in those parts of the composite image where the corresponding images differ in detail. (2) First one picture of a stereoscopic pair is viewed by one eye and then the other picture is viewed by the other eye, the viewings taking place alternately and in rapid succession, as by blinking first of one eye and then the other. The viewer gets the impression of a single, fused picture. A flickering will be noticed in those regions of the composite where the two pictures differ appreciably in detail. The effect is usually obtained without making the viewer blink, by placing the two pictures side by side but separated by a thin partition placed so that each eye of the viewer can see only one picture. The pictures are then illuminated rapidly and alternately.