Objects made of glass and rock crystal, especially when very transparent, must have stimulated the first observations on the possibility of using these materials, properly worked, as instruments to aid human sight, with the awareness of altering natural vision. Several archaeological museums conserve lenses of glass and rock crystal that cover a broad chronological range, from the Minoan-Mycenaean age to the Roman epoch. Admitted in 1766 to visit the royal palace of Portici where finds from Herculaneum and Pompeii were conserved, French traveller Louis Dutens wrote "…in the Cabinet of Antiquities of the King of Naples in Portici, I saw many magnifying glasses and lenses better than those used among our engravers today..."