Nimrud lens
The Nimrud lens, also called Layard lens, is an 8th-century BC piece of rock crystal which was unearthed in 1850 by Austen Henry Layard at the Assyrian palace of Nimrud in modern-day Iraq. It may have been used as a magnifying glass or as a burning-glass to start fires by concentrating sunlight, or it may have been a piece of decorative inlay. Description The lens is slightly oval and was roughly ground, perhaps on a lapidary wheel. It has a focal point about from the flat side and a focal length of about 12 cm. This would make it equivalent to a 3× magnifying glass. The surface of the lens has twelve cavities that were opened during grinding, which would have contained naphtha or some other fluid trapped in the raw crystal. The lens is said to be able to focus sunlight although the focus is far from perfect. Because the lens is made from natural rock crystal, the material of the lens has not deteriorated significantly over time. The Nimrud lens is on display in the British Museum. Interpretation The function of the lens is not clear, with some authors suggesting that it was used as an optical lens