A large drum, painted on the inside with black and white vertical stripes, rotates slowly about its axis. When a visitor focuses on the stripes, the eyes will involuntarily follow the stripes momentarily and then snap back. This can be observed by either watching another visitor’s eyes or closing one eye and placing one finger gently against the eyelid to feel the movement. The involuntary motion can be stopped by focusing on a finger near the drum, but then the stripes are blurred.