instances. We have also said, that the reflection of the heating rays was governed by similar laws to those influencing the rays of light; consequently, by directing a pencil of sunlight upon the surface of a concave mirror, we obtain the maximum of light and heat at the focal point.
Many modern writers give the ancients too little credit for their knowledge of optical principles, and late investigations seem to prove that the old school of philosophers were much more learned in these matters than has been generally supposed. The discovery of a rock crystal double convex lens in an Egyptian tomb of great antiquity is an instance of this. Descartes wrote a little treatise to prove that the stories related of the burning mirrors of Archimedes were pure fabrications, although many Latin authors have described them both as being used by that philosopher and in more modern times; Dion, for instance, who lived in the early part of the sixth century, states that at the siege of Constantinople, Proclus burnt the fleet of Vitalian with mirrors of brass; but the opinion of Descartes seemed to outweigh all other testimony. Buffon, who wished to sift the matter thoroughly, constructed for himself, after many previous experiments on the laws of reflection, a series of mirrors that closely imitated those ascribed to Archimedes. His first memoir, "On the Invention of Mirrors capable of burning at a great Distance," was published in the Transactions of the French Academy of Sciences for 1747. A few years later he combated both theoretically and practically the opinion of Descartes, in a memoir containing an account of an immense number of experiments. Before speaking of the extraordinary effects of burning mirrors, it will be as well to do justice to the predecessors of the learned naturalist we have just mentioned, by quoting a passage from the works of Father Kircher, who, 128 years pre-
