Look steadily at a carpet having figures of one colour, green, for
example, upon a ground of another colour, suppose red, and you will
sometimes see the whole of the green pattern as if the red one were
obliterated; and at other times, you will see the whole of the red
pattern, as if the green one were obliterated. The former effect takes
place when the eye is steadily fixed on the green part, and the latter,
when it is steadily fixed on the red portion.