"As early as the middle of the seventeenth-century burning mirrors and glasses were used for chemical operations such as calcination of antimony. Later, concave mirrors and convex lenses of about one metre in diameter, able to reach temperatures from 1400 to 2000°C at their focus, allowed a decisive progress in the knowledge of the nature of metals. This breakthrough led the mid-eighteenth century chemists, especially Rouelle and Macquer, to reconsider the Philosophers' Stone and the alchemical dream of recomposing gold and silver. Macquer's personal thoughts on the Philosophers' Stone are reproduced here in full. Moreover, the behaviour of gold and ferruginous earths exposed at the focus of Tschirnhaus' lens led Macquer to a hypothesis about the nature of solar rays that came surprisingly close to Homberg's Principle Sulphur at the beginning of the century."