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How many people around the world need glasses and don’t have them? Until surprisingly recently, nobody knew. Now we have an eye-popping answer: 2.5 billion. Many of these people may not be aware that a simple pair of reading glasses could help them to see more clearly. The very first pair of spectacles was probably made in Italy in the late thirteenth century, inspired by the writings of an eleventh-century Arabic scholar. They were a godsend for ageing monks and merchants – and, as Tim Harford explains, they ended up inspiring the invention of the microscope and telescope, too. Producer: Ben Crighton Editor: Richard Vadon