Burning Glass And Burning Mirror, instruments to concentrate the sun's heat. The usual burning glass is simply a double convex lens, which brings the rays of solar heat to a focus at nearly the same point at which it brings the rays of light. Artificial heat cannot in general be brought to a focus by a glass lens; but a lens of rock salt will bring heat radiating from any source to a focus. The use of burning glasses or burning crystal is alluded to by Aristophanes, and several writers declare that Archimedes fired the Roman ships by means of burning mirrors. In the 17th and 18th centuries many experiments were made with burning glasses of immense size. Tschirnhausen made several, some of which are still at Paris, 33 in. in diameter. In 1774 Lavoisier and Brisson superintended the making of a lens 4 ft. in diameter, of two glasses like watch crystals, with various fluids between. This is called Trudaine's lens, from the person who bore the expense. About the year 1800 a Mr. Parker of London made a lens 3 ft. in diameter, which is now at Peking. The heat from these large lenses is intense, and capable of melting any stone or mineral in a few seconds. Equal effects may be obtained from mirrors. Heat is reflected like light, and a concave mirror brings both to a focus.

About 1670 a M. Vilette of Lyons constructed several mirrors of polished metal, from 30 to 50 in. in diameter. Tschirnhausen made one of copper nearly 5 ft. in diameter. Buffbn (who was the first to suggest a lens made of several pieces, afterward brought to perfection by Fresnel, and of great use in lighthouses) made a large reflector of several hundred smaller ones, each 6 in. by 8. With this he set fire to wood at the distance of 210 ft., proving the possibility, though not the probability, of Archimedes having thus burned the Roman fleet. It having been shown that the sun's rays have a heating power partly proportioned to the heat of the place into which they shine, the galvanic flame of a large battery has been made to play through the focus of a large burning glass, and thus the most intense heat ever witnessed has been produced, beyond all reasonable comparison with those temperatures that can be measured by degrees. In all these experiments the most blinding light accompanies the heat, which renders it somewhat difficult to observe the effects.

Priestley's "History of Optics," Bos-sut' Histoire des mathematiques, the Memoires of the French academy for 1777, and Button's supplement to his "Natural History," give further information on this subject.