Definitions

constable-glass

Burning-glass

A burning-glass is a large convex lens that can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition of the exposed surface. They were 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. Burning mirrors achieve a similar effect by using reflecting surfaces to focus the light.

History

The technology of the burning-glass has been known since antiquity. Vases filled with water used to start fires were known in the ancient world, and metaphorical significance was drawn (by the early Church Fathers, for instance) from the fact that the water remained cool even though the light passing through it would set materials on fire. Burning lenses were used to cauterise wounds and to light sacred fires in temples. Plutarch refers to a burning mirror made of joined triangular metal mirrors installed at the temple of the Vestal Virgins. Aristophanes mentions the burning-lens in his play The Clouds (424 BC).

Archimedes, the renowned mathematician, was said to have used a burning glass (or more likely a large number of angled hexagonal mirrors) as a weapon in 212 BC, when Syracuse was besieged by Marcus Claudius Marcellus. The Roman fleet was supposedly incinerated, though eventually the city was taken and Archimedes was slain.

The legend of Archimedes gave rise to a considerable amount of research on burning-glasses and lenses until the late 17th century. Successful recreations have been performed by Anthemius of Tralles (6th century AD), Proclus (6th century) (who by this means purportedly destroyed the fleet of Vitellus besieging Constantinople), Ibn Sahl in his On Burning Mirrors and Lenses (10th century), Alhazen in his Book of Optics (1021), Roger Bacon (13th century), Giambattista della Porta and his friends (16th century), Athanasius Kircher and Gaspar Schott (17th century), the Comte du Buffon in 1740 in Paris, Ioannis Sakas in the 1970s in Greece, and others. Sakas was able to ignite a wooden boat at some distance in only seconds. Buffon, using only 48 small mirrors, was able to melt a 3 kilogram (six pound) tin bottle, and ignite wood from a distance of 46 meters (150 ft). These recreations show the plausibility of Archimedes' achievement.

The pop science TV program MythBusters attempted to model Archimedes' feat by using mirrors to ignite a small wooden boat covered with tar, with only partial success—they found it too difficult to focus light from their hand-held mirrors onto a point small enough to ignite the boat.

Recent excavations at the Viking harbor town of Fröjel, Gotland in Sweden have revealed a small number of rock crystal lenses known as the Visby lenses. These may have been made using pole-lathes. They have an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses effectively concentrate sunlight enough to ignite fires; however it is not known whether they were used for this purpose. Similar technology may have been used in ancient Ireland (the Liath Meisicith) and quite possibly ancient Egypt.

In 1796, during the French Revolution and three years after the declaration of war between France and Great Britain, Étienne-Gaspard Robert met with the French government and proposed the use of mirrors to burn the invading ships of the British Royal Navy. They decided not to take up his proposal.

Chapter 17 of William Bates' 1920 book Perfect Sight Without Glasses, in which the author argues that observation of the sun is beneficial to those with poor vision, includes a figure of somebody "Focussing the Rays of the Sun Upon the Eye of a Patient by Means of a Burning Glass." Needless to say, this is highly dangerous.

Current use

Solar furnaces are used in industry to produce extremely high temperatures without the need for fuel or large supplies of electricity. They employ a large parabolic array of mirrors (some facilities are several stories high) to focus light to a high intensity.

References

Further reading

  • Temple, Robert. The Crystal Sun, ISBN 0 7126 7888 3.

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