A burning-glass is a large convex lens, which can focus the sun's rays on a small area and so ignite materials. Used in 18th century chemical studies for burning materials in closed glass vessels where the products of combustion could be trapped for analysis, the burning-glass was a useful contrivance in the days before electrical ignition was easily achieved.
The technology of the burning glass has been known since antiquity. Aristophanes mentions the burning-lens in The Clouds (424 BC), and Archimedes, the renowned mathematician, was said to have used a burning glass in 212 BC when Syracuse was besieged by Marcellus. Archimedes constructed a burning-glass on a scale of such magnitude that by means of it the Roman fleet was barbequed crispy, though eventually when the city was taken, he was found among the slain. The legend of Archimedes gave rise to a considerable amount of reseach on burning glasses and lenses until the late 17th century.
Recent excavations at the Viking harbor town of Fröjel , Gotland in Sweden have revealed that this technology of fire-starting was known in the Viking Age as well. Rock crystal lenses produced at Fröjel in the 11th to 12th century via turning on pole-lathes have been found that have an imaging quality comparable to that of 1950's aspheric lenses. The Viking lenses quite effectively concentrate sunlight enough to ignite fires.
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