The café wall illusion, in which the straight dividing lines between staggered rows with alternating black and white "boards" appear to be sloped, but in reality are parellel with each other.
This illusion first discovered and described under the name "Kindergarten illusion", and re-discovered by Richard Gregory, a psychology and "Emeritus" Professor of Neuropsychology at the University of Bristol, in 1973.
This effect was observed by a member of Gregory's laboratory, Steve Simpson, in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol. It is a variant of the shifted-chessboard illusion originated by Hugo Münsterberg.
In the first and quite successful attempt at its deconstruction the illusion was ascribed largely to irradiation, the light spread from dark to bright zones in the retinal image, and indeed the image disappears when black and white are replaced by different colors of the same brightness (isoluminant). But a component of the illusion remains even when all optical and retinal components are factored out. Contrast polarities seem to be the determining factor in the tilt's direction.