The idea of a fluid lens whose focal plane can be mechanically adjusted goes back at least to an 1866 patent awarded D. A. Woodward, a Baltimore inventor. Since then there have been a variety of attempts to commercialize the technology, but none have met all the criteria for success. The lens must be thin, light, durable and easily adjustable. Several years ago Dr. Kurtin had his epiphany. A magnetically attached removable front lens would make for a compact and durable system.

Others have done research on approaches that do not involve a fluid, like systems of lenses and electro-optical technologies. In 1964, for example, Luis W. Alvarez, who in 1968 won the Nobel Prize in physics, designed a two-part lens that changed focus by sliding two glass components of opposing “saddle-back” shapes across each other.

Several international efforts are under way to adapt both fluid lenses and the Alvarez approach to the 1.3 billion people at the bottom of the economic pyramid that the World Health Organization estimates as having no access to eyeglasses. Both the Center for Vision in the Developing World in Oxford, England, and U-Specs in Amsterdam are working on glasses that can be distributed at a fraction of the price that glasses cost in the developed world.

The TruFocals eyeglasses, which sell for $895, are the first to become commercially available in the United States. The glasses consist of a lens that is comparable in thickness to that of commercial eyeglasses, but which has three components: a back glass, a fluid-filled inner membrane that is essentially a piece of plastic-wrap-like material stretched across a ring whose surface curvature can be altered mechanically, and an outer prescription lens attached with magnets to the eyeglass frame. To change the focus, the user moves the slider on the bridge.

The TruFocals lens must be round to ensure that the curvature of the interior lens is correct, but the company’s founders do not think the shape is a fashion obstacle. TruFocals come in a unisex frame with a variety of colors.

Even if TruFocals are not accepted as eyewear for all uses, they may find a market for a diverse set of wearers, like office workers who work at computer screens but who must also read books, magazines and newspapers, or people who have medical conditions that cause their vision to change.

“This is not going to break open the whole market, but they don’t need to break open the whole market,” said Eli Peli, a professor of ophthalmology at Harvard Medical School and an investor in TruFocals. For the moment TruFocals has the market to itself, but it may have competition next year. PixelOptics, based in Roanoke, Va., is pursuing an electro-optical approach, which will use a technology that is similar to an LCD display to change the refractive index of a transparent component that the company will embed in conventional lenses.