This is an illusion pointed out to me by Tom Tompkins of the San Francisco Exploratorium in August, 1998. The walls of the new Exploratorium bathrooms are tiled with this pattern. All three panels above have identical tile spacing. The only difference is the color of the grout between the tiles. Note that if the grout is white, as in the left panel, the tiles appear to be in straight horizontal columns. The "wavy" effect becomes stronger with darker grout. For my eyes (and brain) 60% black grout creates the strongest illusion.
Below are some photographs of the bathroom wall.
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