You are near-sighted. You wear glasses. You have to go to opticians to have your lenses changed. Josh Silver, a professor of physics at Oxford University, may be able to help you out. He has invented a pair of special glasses. Wearers can correct their vision by adjusting the lens themselves. The glasses are based on a simple principle: The fatter a lens, the more powerful it becomes. Inside the plastic lenses are two clear bags filled with fluid. The wearer can add or reduce the amount of fluid with a syringe. It changes the power of the lens. The invention may help near-sighted people in the developing world greatly. In Britain there is one optometrist for every 4,500 people. In sub-Saharan Africa the ratio is 1:1,000,000. So far, Professor Silver has made 30,000 pairs of his glasses and given them away in 15 countries. He plans to offer glasses to a billion of the world’s poorest people by 2020.