Mach bands
(well known to artists of the 19th century)

All of the blue stripes are of uniform brightness.  If you don't believe it, test it by covering the transistion to the next stripe with the edge of a piece of paper.

The "Mach bands*," that illusion of darkening or lightening as you get close to an edge, is the result of a mechanism in your eye that enhances edges.  After your rods and cones detect the light, but before the message gets sent to the brain, two layers of nerve cells process the information further.  These "horizontal" cells inhibit the response of adjacent cones, and that inhibition causes the increased contrast you "see" at the edge.

*Ernst Mach,  The Analysis of Sensations, 1914.
 

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