Kingdom FAA, Yoonessi A & Gheorghiu E (2017) Leaning Tower Illusion. In: Shapiro A & Todorovic D (eds.) The Oxford Compendium of Visual Illusions. Oxford: Oxford University Press, pp. 221-226. https://global.oup.com/academic/product/the-oxford-compendium-of-visual-illusions-9780199794607?cc=gb〈=en
First paragraph: Two identical side by side images of the Leaning Tower of Pisa, photographed from below, appear to rise at different angles (Kingdom, Yoonessi, & Gheorghiu, 2007a,b; see Fig. II.21- 1). Although the name of the illusion is a play on the name of the famous building, the illusion works for any image of a receding object. The illusion was discovered by accident in 2007 during a psychophysics (behavioral) experiment conducted by two of the authors. Test observers were shown color- manipulated pairs of images of the same scene and noticed that the pair of Pisa towers appeared different in form not just color. So striking was the effect that it was assumed that the two images had been taken from different photographs. The illusion won an international visual illusion competition in the same year (Kingdom, Yoonessi, & Gheorghiu, 2007c) and has subsequently been featured in numerous books, journals, talks, newspaper articles, calendars, scientific websites, and blogs. Part of the attraction of the leaning tower illusion is its simplicity— one only has to place two identical copies of a photograph of any receding object (e.g., buildings photographed from below, railway lines etc.) side by side to obtain the illusion.