If we look successively through the eye holes of two large boxes painted respectively black and white on the inside, and one illuminated faintly, the other powerfully, so that the quantity of light received by the eye in each case is the same, and if we contrive that inside the boxes there shall be no shadow or lack of uniformity in the painting, they then become indistinguishable, and in each case we see only an empty space permeated by grey.