The Cafe Wall Illusion
The famous Cafe Wall Illusion (see Richard Gregory, Eye and Brain, 5th Edition, Princeton University Press 1997, page 211) develops when a staggered arrangement of light and dark bricks are separated by thin lines of 'mortar' whose brightness is intermediate between the brightness of the light and dark bricks. This creates a powerful impression of criss-crossing 'slant' in the perfectly horizontal 'mortar' lines. The illusion can be produced by simply tiling the brick seen below.
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The Cafe Wall Illusion is very sensitive to the details of its layout, as we can see by tiling the two bricks seen below. The illusion disappears if the 'mortar' lines between the bricks are not intermediate in brightness between the dark and light bricks, and disappears, or even reverses, if the horizontal 'mortar' line is made thicker.
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