The Staircase
Nine bars of grey, each one step lighter than the last.
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Nine bars of grey, each one step lighter than the last.
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Edges, sharpened
Every bar is a single flat grey. Where two bars meet, the lighter one looks lighter still along the edge and the darker one darker still — thin bands that are not in the picture.
The physicist Ernst Mach described them in 1865. Here the old explanation holds up: cells in the retina are held back by their neighbours, which exaggerates any step in brightness, and so the visual system sharpens edges before anything else sees them.