Linear or Circular
This pendulum and filter experiment demonstrates that a pendulum swinging in one plane ‘appears to have an elliptical path when viewed with a neutral density filter in front of one eye. This was first noted by Pulfrich and the phenomenon is called a Pulfrich effect. This illusion is caused due to the perceptual mechanism in the eye. The use of neutral density filter in front of one eye creates distortions in the perception of the depth, size, velocity, and position.