Bowed Lines
Two red lines across a burst of rays. Are they straight?
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Two red lines across a burst of rays. Are they straight?
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Rays that bend
Both red lines are ruler-straight. Near the centre the rays cross them at shallow angles and further out at steep ones, and each crossing angle is misjudged a little; along the line those small errors change smoothly, which the eye reads as the line itself curving away from the centre.
Ewald Hering published it in 1861. One idea is that the rays look like the view when you move forward — lines streaming out from the point you are heading for — and the brain predicts a scene a fraction of a second ahead, bending what it expects to pass you.