Hermann Grid Illusion

Hermann Grid jpg is here
While he was reading a book on sound by John Tyndall, L. Hermann (1870) saw gray spots in the intersections of spaces among the figures that Tyndall had arranged in a matrix. The same intensity light is reflected/transmitted all the way along the white spaces in the grid and yet the intersections appear gray. What occurs in your visual system that could account for the illusory experience of gray spots?

You can view spots among the colors in a palette and compare your experience to what you see here.

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