The oval lens is ground in such a manner that beams of light passing through it are bent to concentrate them in one spot on an object placed beneath it. The intense light will raise the temperature of the object until it reaches the incendiary point. The lens is housed in a tough leather frame, 3/8-inch wide, that is extended at one point to provide a short, spatular tab by which the glass can be held. The leather frame has an incised line running round its edges; and is pierced by a hole, possibly for a hanging cord.
Fewer than 10 of such lenses from the 18th century are known to survive.