blindspot

Instructions: Start with your face six inches away from the screen. Close your right eye and focus your left eye only on the X. Now move away from the screen slowly, and at one point the O will disappear. This will happen when the screen is about 14-18 inches away (3x the width of this diagram). As you move further away, the O will reappear. If you close your left eye and look at the O, the X will disappear over a similar range.