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Hermann Grid
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Hermann Grid

In the classic Hermann Grid (shown at left), fuzzy dark patches appear at all of the intersections except the ones you are looking straight at. When the black and white regions are inverted, as in the right figure, the fuzzy patches appear white instead of black.

ReferenceReferences

[1] L. Hermann, "Eine Erscheinung simultanen Contrastes", in Pflügers Archiv für die gesamte Physiologie, vol. 3, pp. 13-15, 1870.
[2] H. K. Hartline, H. G. Wagner, and F. Ratliff, "Inhibition in the Eye of Limulus", in The Journal of General Physiology, vol. 39, pp. 651-673, 1956.
[3] J. Geier, L. Bernáth, M. Hudák, and L. Séra, "Straightness as the main factor of the Hermann grid illusion", in Perception, vol. 37, pp. 651–665, 2008.