Cafe Wall Illusion

The cafe wall illusion, also known as the Munsterberg illusion or the kindergarten illusion, is a geometrical-optical illusion in which the parallel straight lines dividing staggered rows of alternating dark and light rectangles, resembling rows of bricks or tiles, appear to slope even though they are perfectly parallel in reality. The effect has been rediscovered more than once: a version was first described by Hugo Munsterberg in 1894, then described again as the kindergarten illusion by A. H. Pierce in 1898, and finally given its current name in 1973 by Richard Gregory, after a member of Gregory's laboratory, Steve Simpson, noticed the effect in the tiled wall of a cafe at the bottom of St Michael's Hill in Bristol. The illusion is typically constructed with a thin strip of intermediate-toned mortar between each row's dark and light bricks; researchers largely trace the effect to the irradiation illusion, the apparent tendency of white areas to look larger than black ones, since the illusion disappears when the black and white bricks are replaced with two different colors of equal brightness, though a smaller residual effect persists even once purely optical and retinal factors are ruled out, and the direction of the apparent tilt depends on the contrast polarity of the bricks.