A company called Adlens out of the UK specializes in a product called adjustable-lens eyeglasses, driven by the goal of helping correct vision easily in developing countries. From their site:
The lens is a hollow chamber with a thin, clear, strong plastic sheet stretched across inside.
The variable lens power comes from a clear, high refractive index fluid. The fluid is pumped into (or out of) the chamber between the plastic-like sheet and the lens. As the amount of oil increases, it pushes on the plastic sheet, changing its curvature. The more it curves, the more the lens power increases.
When the fluid is pumped back out, the lens curvature flattens, reducing the power. The base hard lens has a negative power, meaning that with little or no oil, objects are demagnified.