Judd Illusion

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Judd Illusion

If the middle of a segment is marked by a short evenly (with regard to the two halves) spaced rectangle or a dot, the halves appear to have different lengths depending on the direction in which the arrow heads point. The illusion disappears or at least becomes less pronounced when the marker is replaced with an arrowhead.

References

The Mind's Eye, Readings from SCIENTIFIC AMERICAN, W. H. Freeman and Company, 1986

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