p164fig04.40 The Koffka-Benussi ring.  See the text for details.<BR>|| p164c2h0.25 "... [left image] The luminance of the ring is intermediate between the luminances of the two background regions. Its perceived brightness is also between the brightnesses of the two background regions, and appears to be uniform throughout. The right image differs from the left only in that a vertical line divides the two halves of the ring where it intersects the two halves in the background. Although the luminance of the ring is still uniform throughout, the two halves of the rig now have noticeably different brightnesses, with the left half of the ring looking darker than the right half. How can drawing a line have such a profound effect on the brightnesses of surface positions that are so far away from the line? ..."