A glass case in the Assyrian section of the British Museum contains a piece of rock crystal formed into the shape of a plano-convex lens 1 1/2 inches in diameter and 9/10 inch thick. This was discovered in the ruin called Nimroud. It gives a focus of 4 1/2 inches. According to Sir David Brewster, this lens has been designed for magnifying purposes. The date is about 700 B. C.
Anonymous, "Antiquity of the Lens," Scientific American, vol. 69, no. 7 (August 12, 1893), p. 104.