The Pulfrich Effect: Discussions & Text Explanations

Loeb 1918, p. 72:  Fly with one occluded eye can climb only in a spiral

The Pulfrich Effect is an optical illusion in which an object which is moving in a plane parallel to the viewer's forehead seems to move out of that plane and to approach (or recede from) the viewer. To cause the illusion, it is necessary only to dim the light reaching one eye, for example by means of one sunglass lens or a smoked piece of glass. This illusion is stereoscopic: It occurs only when the moving object is being viewed with both eyes.

The classical demonstration of the Pulfrich Effect is by a swinging pendulum. The pendulum is swung back and forth; 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. Dimming the light equally to both eyes does not cause the illusion. If the pendulum already is swinging in an ellipse, dimming the light to one eye changes the shape of the ellipse.

Real pendulum path
Apparent pendulum path
The real path of the pendulum is in a plane.
The Pulfrich pendulum appears to move in an elliptical path, toward and away from the observer.

Alfred Lit: Adapted from Lit (1960).
J. M. Williams: Adapted from doctoral dissertation (1980).

Historical Note:
Lit's apparatus for presenting moving targets for the study of stereoscopic vision is based on a design often described as the Howard-Dolman apparatus. The actual device used for Lit's Pulfrich-related studies was constructed originally at Pupin Physics Laboratories, Columbia University in the City of New York, in about 1949.
Mechanical details of the Scotch Yoke assembly for target movement are available at this web site.

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The Pulfrich Effect, SIU-C. Last updated 2004-09-30