The Pulfrich effect (named after Carl Pulfrich) is a well described visual stereoillusion observed when a swinging pendulum bob is viewed through a neutral density filter in front of one eye.¹ Although the bob is moving in a frontal plane, the path seems elliptical. The effect arises from the fact that dimming a stimulus with a neutral density filter slows signal conduction velocity between the eye and the cortex. The visual cortex interprets this as a false depth or disparity cue as the object appears in a different location in the two eyes.²

Suppose an object happened to be moving from left to right, at a constant distance from the observer. Then, no matter what the (fixed) distance to which the eyes happened to be converged, any given instant, the dual images of the object, reversed by the optics of the eyes, would be moving right to left with some particular retinal disparity proper to the real distance between the object and observer.

Now, if a filter happened to be before the left eye (the Pulfrich effect), the response in the right eye effectively would be advanced somewhat. The advance would change the percept so that, …