When you stare at the cross for at least 30 seconds, you see three illusions:

• A gap running around the circle of lilac discs;
• A green disc running around the circle of lilac discs in place of the gap; and
• The green disc running around on the grey background, with the lilac discs having disappeared in sequence. — Lilac Chaser, Wikipedia

Elsewhere Other-Accessible…

• Troxler’s fading at Wikipedia