Page "Pulfrich effect" Paragraph 2

In the classic Pulfrich effect experiment, a subject views a pendulum swinging in a plane perpendicular to the observer's line of sight.

When a neutral density filter ( a darkened lens — typically gray ) is placed in front of, say, the right eye the pendulum seems to take on an elliptical orbit, appearing closer as it swings toward the right and farther as it swings toward the left, so that if it were to theoretically be viewed from above, it would appear to be revolving counterclockwise.

Conversely, if the left eye is covered, the pendulum would appear to be revolving clockwise-from-top, appearing closer as it swings toward the left and farther as it swings toward the right.