A burning glass or burning lens 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. Burning mirrors achieve a similar effect by using reflecting surfaces to focus the light. 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.
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Burning glass in the context of Lens
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis. Lenses are made from materials such as glass or plastic and are ground, polished, or molded to the required shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called "lenses", such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.
Lenses are used in various imaging devices such as telescopes, binoculars, and cameras. They are also used as visual aids in glasses to correct defects of vision such as myopia and hypermetropia.
Burning glass in the context of Sacred fire of Vesta
The sacred fire of Vesta was an eternal flame in ancient Rome dedicated to the goddess Vesta, protector of the hearth, home, and family. It was tended by the Vestal Virgins, a unique priesthood that began with two members, later increased to four, and eventually to six. Chosen by lot from patrician families, the Vestals served for thirty years, during which they ensured the flame never went out and performed rituals linked to domestic life. These included the ceremonial sweeping of the temple on June 15 and the preparation of sacred food for major festivals. As Vesta embodied the hearth, the flame symbolised both the life of every Roman household and the vitality of the state. Its eternal burning was believed to guarantee the goddess’s protection and the enduring strength of Rome itself.
The fire was renewed every year on the Kalends of March. Plutarch's (c. 1st century AD) Parallel Lives records the Vestal Virgins’ use of burning mirrors to relight the fire: