A well-known optical illusion: there are two horizontal parallel lines of equal length, the top one bounded by outward-facing arrows and the lower one by inward-facing arrows. The lower line looks longer, though it is the same length. The illusion was described by Franz Carl Müller-Lyer, a German sociologist, in 1889.
Used on page 113
Links:
Wikipedia: The Müller–Lyer illusion