Spectacles have been around for centuries, and have a huge impact on many people's quality of life. So why is it estimated that more than two billion people aren't aware that they need them? Tim Harford considers the difference that seeing clearly makes to the world.
Theories of Vision from Al-kindi to Kepler, 1981, David C. Lindberg, University of Chicago Press, p 209-210 and p 86
Microscopes and Telescopes, Rebecca Stefoff, 2007, Marshall Cavendish, New York, p 12-13 and p14-16.
Artifacts from Medieval Europe, James B. Tschen-Emmons, 2015, ABC-CLIO, Santa Barbara, p 260.
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A History of Reading, Steven R. Fischer, 2004, Reaktion Books, London, p 186.