Stare at the image for about half a minute without moving your eyes and watch as it gradually disappears.
f you stare at the center cross for a couple seconds you will begin to perceive a green disco going around the circle of magenta discs
https://tse3.mm.bing.net/th?id=OIP.tbX8eY7Em3scBTnSxI9vWgEsDP&pid=15.1&P=0&w=244&h=170