Before we quit the subject of glass we must not forget to mention a very curious object found by Layard at Nimroud, in the palace of Assurnazirpal, and in the neighbourhood of the glass bottle and the two alabaster vases on which the name of Sargon appears. It is a lens of rock-crystal; its convex face seems to have been set up, with some clumsiness, opposite to the lapidary on his wheel. In spite of the imperfect cutting, it may have been used either as a magnifying, or, with a very strong sun, as a burning, glass.[373] The fineness of the work on some of the cylinders, and the minuteness of the wedges on some of the terra-cotta tubs, had already excited attention, and it was asked whether the Assyrians might not have been acquainted with some aid to eyesight like our magnifying glass. It is difficult, however, to come to any certain conclusion from a single find like this; but if any more lenses come to light we may fairly suppose that the scribes and lapidaries of Mesopotamia understood how thus to reinforce their eyesight. In any case it is pretty certain that this is the oldest object of the kind transmitted to us by antiquity. § 2. _Metallurgy._ Even at the time to which we are carried back by the oldest of the graves at Warka and Mugheir, metallurgy was already far advanced in Chaldæa. Tools and weapons of stone are still found in those tombs in great numbers;[374] but side by side with them we find copper, bronze, lead, iron, and gold. Silver alone is absent.