We describe and test a simple quantitative schlieren method for measuring the slope of an unsteady free liquid surface along a straight segment. The optical system, based on an anamorphic element placed at the focal plane of a standard objective, forms a distorted image of the illuminated segment on a screen. The abscissa of the resulting bright curve is proportional to the coordinate along the segment and the ordinate to the longitudinal slope at the corresponding coordinate, provided that the

more »

... transverse slope component is negligible. The method is useful to probe axisymmetric flows along a diameter or flows in straight channels along the axis.