Gray Step I  Horse's Tail
Edges are important in defining what we see. When you lift the rope tail, you see two separate squares, one significantly darker than the other. When the rope tail coveres the edge, you can't tell the difference between the two sides. The two squares are really identical. There is only a difference at the point where they meetthe edge. Each square is shaded lighter to darker, from left to right. In the middle, the dark edge of one square meets the light edge of the other. Your eye concentrates on the boundary and assumes the rest of the square is the same.

