77 Optical Illusions & Visual Phenomenaより"Lilac Chaser"Watch the X in the middle very closely. You should start to see a green dot that rotates around the circle - this dot is an illusion; then you should see see the purple dots disappear.... but they haven't really gone. It is an after image effect, sometimes called a 'negative retinal afterimage' - move your head slightly, and the dots will reappear... amazing or what?オススメ（HP:http://www.michaelbach.de/ot/index.html）