Pulfrich's illusion

Pulfrich's illusion is seen when a simple pendulum is viewed pendicular to the plane of oscillation with one eye covered by a filter. The effect is more pronounced with a faster moving pendulum.

The image from the filtered eye is slightly delayed compared to that from the unfiltered eye. The two images reaching the brain were generated in the retina/brain system at different times, the they are interpreted as having occurred simultaneously. The brain converts the percieved differences in time as three-dimensional data. The pendulum appears to be conical.

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