Height differences in the surface of a layer of a turbid, viscous liquid are marked by the intersection of the surface by a ribbon of light. The pattern found in this way is photoelectrically scanned by a rotating drum provided with a series of holes, forming a helix on its cylindrical surface, and a photomultiplier. An electronic system gives a magnified and super-elevated image of the profile of the liquid layer on the screen of a cathode-ray oscilloscope. In this way, a steady observation of the decay of the height differences is feasible.