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You can make a set of keys appear to pass through your arm or hand. The Pulfrich illusion, visual sampling rates, and fusions from persistence of vision explain the effect.

You need to create a difference in the light intensities entering the viewers' eyes. The safest way to create the intensity difference uses filters, such as one lens from sunglasses. Viewers wear the lens over one eye while looking with both eyes. The viewers should be seated in a stationary chair because differences in light intensities in each of the eyes will produce depth distortions.

Make a pendulum using shoelaces or string and a set of keys. Swing the keys in a single plane, back and forth in front of you. Two-thirds of people will see the keys moving in an elliptical pattern. The direction of movement of the keys through the elliptical pattern is predictable. With less intense light entering the left eye than the right eye, the keys will appear to swing in a clockwise direction. With less intense light entering the right eye than the left eye, the keys will appear to swing in a counterclockwise (or anticlockwise) direction.

Following the instructions to this point, you will have created a version of the Pulfrich illusion. Pulfrich, for whom the effect is named, and his assistant Fertsch, sought explanations for problems in the use of range finder telescopes, stereocomparators, and other stereo viewing devices. The devices were perfectly aligned by physical measurement, but usage problems appeared. The pair of investigators traced the usage problems to intensity differences of light entering the two eyes. Pulfrich never experienced the illusion named for him because he was a monocular viewer.

You can create many variations of the Pulfrich illusion. For instance, view a 'snowy' television screen with filtering and the snowy dots on the screen will appear to move around a cylindrical pattern. You can also use a low intensity light source, such as a flashlight, to add light to one eye rather than filtering one eye. BE CAREFUL NOT TO SHINE HIGH INTENSITY LIGHTS INTO YOUR OR OTHERS' EYES!

To create the illusion of keys passing through your hand, allow people to view the Pulfrich illusion for a couple of minutes. Over this time, many people report that the effect increases in strength. Then time the movement of your hand from your body, obliquely toward the plane of pendulum movement, so that your hand arrives just behind the plane of pendulum movement just prior to the keys passing in front of your hand. Withdraw your hand back toward your body and repeatedly advance your hand in timing with the swing of the pendulum. To most viewers, the keys will appear to pass right through your hand or arm. The place on your body that the keys appear to pass though will be determined by the strength of the illusion that you have created for your viewers.

• What accounts for differences in strengths of the illusion?
• What explains some people seeing the illusions and other not seeing them?
• Can you develop 3-D displays of xray photos, movies, television, etc. without requiring viewers to use special glasses or filters?