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Eternal Light

Augustin Fresnel spent much of his life in anonymity supervising road construction for the French government, but as the inven- tor of the Fresnel lens (pronounced “Fre-nel”), the physicist and engineer’s name is forever linked to lighthouses.

Fresnel discovered that by combining many individual pieces of glass, even the insubstantial light of an oil lamp could be refracted into a powerful beam. His lenses were produced in six sizes or orders.The most powerful irst-order lenses might contain more than 1,000 individual prisms and pieces of optical glass—with the result that a single light could be seen by ships up to 42 km away.

There are two main types of Fresnel lens: drum lenses, and bull’s eye lenses.The former were stationary, but bull’s eye lens- es rotated using various mechanical systems. Each light’s unique pattern was determined by the number of bull’s eyes as well as by the rotation speed. With a chart showing the location and pattern of nearby lights, a ship’s pilot could plot a precise and safe course.

Before electricity, a crank and weight were used to turn the

FEBRUARY 2009

San Francisco to Puget Sound north of Seattle

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