(30 minutes or less)
You can use the image provided as a separate page, here, or make your own gray step by following these instructions.
Punch two holes 4 inches (10 cm) apart near the edge of a sheet of white paper and set it aside.
Put the large sheet of graduated paper in front of you with the dark side to the left. Cut a strip 2 inches (5 cm) wide from the right side of the paper (the lightest side). Throw this strip away.
Now cut a 4 inch (10 cm) wide strip from the new right side. Cut this piece in half so that you have two shorter but identical pieces.
Place the light side of one piece next to the dark side of its twin. Mount the pieces on cardboard or masonite with glue or tape. Attach the horse tail above the boundary between the two pieces so that it hangs down and covers the boundary.
(5 minutes or more)
In this Snack, a fuzzy cord (the "horse tail") is used to obscure the boundary between two gray areas. You see one uniform gray area when the horse tail is in place, and two different gray areas when the horse tail is removed. But you never see the truth: Both gray areas are really identical to each other and grade from light gray at one edge to dark gray at the other. In general, your brain ignores slight gradations in gray level.
If you are using the image in this book, just place a pencil or your finger over the boundary between the gray halves. What do you see? If you are using your own construction, position the horse tail so that it covers the boundary and ask your friends what they see. Most people will see a uniformly gray piece of paper with a rope hanging down the middle.
Lift the tail and ask again. Most people will see two uniform areas, each a different shade of gray.
To show that the two sides are identically shaded areas, hold the punched piece of paper over the gray areas, with one hole on each side of the boundary. The two areas viewed through the punches will always be the same shade of gray.
Actually, the two rectangles are exactly the same. At the right edge both rectangles are light gray. Both become darker toward the left. Where the rectangles meet, the dark part of one rectangle contrasts sharply with the light part of the other, so you see a distinct edge. When the edge is covered, however, the two regions look the same uniform shade of gray.
It is difficult to distinguish between different shades of gray or shades of the same color if there is no sharp edge between them. This is true even though a sensitive light meter would show that the different shades are reflecting different amounts of light to your eyes. Your eyes do not lack the necessary sensitivity to detect the difference: if there is an edge between the two shades, the difference is obvious.
Your eye-brain system, however, condenses the information it obtains from more than a hundred million light-detecting rods and cones in the retina in order to send the information over a million neurons to your brain. Your eye-brain system enhances the ratio of reflected light at edges. If one region of the retina is stimulated by light, lateral connections turn down the sensitivity of adjacent regions. This is called lateral inhibition. Conversely, if one region is in the dark, the sensitivity of adjacent regions is increased. This means that a dark region next to a light region looks even darker, and vice versa. As a result, your visual system is most sensitive to changes in brightness and color.
When the horse tail is absent and the normal boundary is visible, lateral inhibition enhances the contrast between the two shades of gray. The bright side appears brighter and the dark side darker. When the tail is in place, the boundary between the two different grays is spread apart across the retina so that it no longer falls on adjacent regions. Lateral inhibition then does not help us distinguish between the different shades, and the eye-brain system judges them to be the same.
Another easy Gray Step demonstration can be constructed out of paint sample pieces from the hardware store. Some paint brands provide good sized individual color chips. Experiment with how different two colors can be and yet appear the same when the boundary between them is obscured.
A good source of background reading is Seeing the Light by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986).