Pulfrich Phenomenon demonstrated with GeoGebra applet.

The Pulfrich phenomenon or effect is a psychophysical perception related to the apparent spatial disparity between the two eyes when the sight of one of the eyes is purposely passed through a filter. The effect was observed in an experiment with a swinging pendulum that was first performed in 1922 by a German physicist called Carl Pulfrich. The pendulum is swung back and forth and its movement is observed with both eyes. Care must be shown that the movements of the pendulum stay in the same plane parallel to the viewer's forehead, who is making the observation. When viewed normally with both eyes, the pendulum just seems to swing back and forth. When a filter is placed in front of one eye, the pendulum suddenly seems to be swinging in an ellipse parallel to the floor. In other words, the pendulum should appear to be tracing an oval path. This is an optical illusion. Dimming the light equally to both eyes does not cause the illusion. You can observe the phenomenon by downloading the ggbfile and clicking at the slider t Animation On.
E1 and E2 represent the eyes. GP2 or GP1 represent the apparent pendulum with its string. GW1W2 is the plane in which the movement is performed. In the slider bar activate Animation On and watch the apparent path of the pendulum P. The true path of the pendulum P is in the segment W1-W2, but when a filter is put in front of the right eye the apparent path is oval (elliptic) traced by the image P2 of P when the the pendulum movement is from right to left (the path looks beyond the mentioned plane of the pendulum swing), and it is traced by the image P1 of P when the pendulum movement is from left to right (the path looks on our side the mentioned plane of the pendulum swing). 1. Try to reveal how the system is constructed. 2. Try out the experiment yourself to confirm theis phenomenon. Build the pendulum in a room by hanging it on the ceiling using a string as long as the object be at the height of your horizontal sight view. Swing it and stay about 2m away to watch the movement while you have put in front of one of your eyes a sunglass lens or a smoked piece of glass.