Burning Glass, a convex lens of large size and short focus, used for causing an intense heat by concentrating the sun's rays on a very small area. The larger the circular area of the lens and the smaller the area on which the rays fall, the greater is the effect produced. Concave mirrors have been used for similar purposes, and are also called burning glasses. Their power was well known to Archimedes, and it is mythically stated that by their aid he burned a fleet in the harbor of Syracuse, 214 B.C.
Entry from Everybody's Cyclopedia, 1912.