“…The fact that the illusion is more pronounced in a complex landscape and scarcely noticeable over an open, level field is consistent with reports on the Pulfrich pendulum illusion, which is not noticeable unless objects other than the pendulum are in the visual field (2,3). The dependence of the illusion on overall illumination, with less distortion in full sunlight, corresponds with quantitative data on the Pulfrich pendulum illusion (6,7).…”