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Some 30,000 pairs of his spectacles have already been distributed in 15 countries, but to Silver that is very small beer. Within the next year the now-retired professor and his team plan to launch a trial in India which will, they hope, distribute 1 million pairs of glasses. The target, within a few years, is 100 million pairs annually. With the global need for basic sight-correction, by his own detailed research, estimated at more than half the world’s population, Silver sees no reason to stop at a billion.

If the scale of his ambition is dazzling, at the heart of his plan is an invention which is engagingly simple. Silver has devised a pair of glasses which rely on the principle that the fatter a lens the more powerful it becomes. Inside the device’s tough plastic lenses are two clear circular sacs filled with fluid, each of which is connected to a small syringe attached to either arm of the spectacles.

The wearer adjusts a dial on the syringe to add or reduce amount of fluid in the membrane, thus changing the power of the lens. When the wearer is happy with the strength of each lens the membrane is sealed by twisting a small screw, and the syringes removed. The principle is so simple, the team has discovered, that with very little guidance people are perfectly capable of creating glasses to their own prescription.

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