In general your eyes and brain ignore slight gradations in gray. Material 2 Exploratorium Gray step Postcards. Or use the image above. Assembly Cut one of the gray steps in half down the middle, trim the black regions off the edges. Most often they will say they see a pencil covering a gray rectangle.
People now report that they see the page divided into two gray halves one lighter than the other.
Then move the two halves of the optional gray step so that one is lined up with its bottom above the top of the first to show that both halves are identical. Then move the right gray area to the left. The bright one now becomes the dark one. or optionally Use the viewer to show that the right and left halves are identical shading from light at the left to darker on the right. A photometer would show that the gray regions change darkness by a few percent from one side to the other, your eye does not see this change. The eye is sensitive to changes in gray level if they are great enough or if they occur over a short distance.