p089fig03.02 What do you think lies under the two grey disks? (on a checkers board)<BR>||  p089c1h0.55 "... As your eye traverses the entire circular boundary (Howell: of a grey disk on a checkerboard), the contrast keeps flipping between light-to-dark and dark-to-light.  Despite these contrast reversals, we perceive a single continuous boundary surrounding the gray disk. ...".