Measurements were made of the perceived contrast that results from the Cornsweet illusion as a function of the edge contrast and edge width. The physical contrast was varied from 3% to 5%, and the visual extent of the Cornsweet edge varied from 0.1 degrees to 3.0 degrees on each side. Results show that, for low-contrast (less than 20%) and wide edges, the Cornsweet edge yields as much contrast as a real-step lumininance transition with the same edge magnitude. At high contrast (30%-40%) the illusion is reduced relative to a real step, regardless of whether the illusion consists of a conventional Cornsweet edge or an additive combination of a real step and a Cornsweet edge. As a general rule, wide edges yield more contrast than narrow edges. The results are interpreted in terms of the visual describing funtion; a nonlinear model is proposed. The correspondence between the predictions of the model and the perceptual measurements on the Cornsweet edges is good.