  Hermann discovered the grid illusion in 1870, but its cause has remained a
mystery for more than 150 years. In 1960, Baumgartner proposed a hypothesis for
the illusion based on neural receptive fields, but Geier presented a
counterexample in 2008. In 1994, Spillmann devised the scintillating grid
illusion, an improvement on the Hermann grid illusion. I propose that a
hypothesis involving blind spots (optic discs) can significantly contribute to
unraveling the mystery of the grid illusion.
