The impossibly twisted Pulfrich pendulum.
Two Pulfrich pendulums swinging in opposite phase present impossible twisting effects and also size-constancy effects.
D003867:Depth Perception / Q000502:physiology; D006801:Humans; D007088:Illusions / Q000502:physiology; D009039:Motion Perception / Q000502:physiology; D009799:Ocular Physiological Phenomena; D009903:Optical Illusions / Q000502:physiology; D012858:Size Perception / Q000502:physiology