Humans are fascinated by visual illusions, which occur when there is a mismatch between the pattern of light that falls on the retina, and what we perceive. Before books, films, and the internet allowed illusions to be shared widely, people were captivated by illusions in nature. Indeed, it is here that the long history of the study of illusions begins. Both Aristotle and Lucretius described motion illusions following observation of flowing water.
Aristotle observed pebbles beneath flowing water for some time, and noticed that afterwards pebbles beside the water appeared to be in motion. Lucretius, meanwhile, looked at the stationary leg of his horse when in the middle of a fast flowing river and noted that it seemed to be moving in the opposite direction to the flow. This is called induced motion and it has long been observed when clouds pass the moon – the moon can seem to move in the opposite direction.
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