Grey Ghosts
A grid of black squares and white streets. Something grey keeps turning up at the crossroads.
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A grid of black squares and white streets. Something grey keeps turning up at the crossroads.
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Ghosts that only appear where you are not looking
Nothing grey is drawn: every crossing is plain white. Grey smudges appear at the crossings you are not looking at straight on, and vanish from the one you look at.
Ludimar Hermann described the grid in 1870, and for a century the textbook explanation was cells in the retina being held back by their neighbours. That explanation predicts the spots should survive when the streets are made wavy — and they do not, which points instead to cells in the visual cortex that respond to straight edges and bars.