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 previously, experimented in this direction with great patience and perseverance, and tried to prove that the stories related of Archimedes were true. “The larger the surface of a mirror,” says this philosopher (who, like Huyghens, was a practised astronomer), “the more light it reflects from the objects opposite to it. If it is only a foot square, it will throw a square foot of light upon any wall or screen placed before it. Experiment shows that this light is composed of an infinite number of rays reflected from different points on the surface of the mirror. Direct the rays from a second mirror upon the same place as those from the first, and the light and heat will clearly be doubled. They will become trebled if you direct the rays from a third mirror upon the same spot, and so on _ad infinitum_. In order to prove that the intensity of the light and heat is in direct proportion to the number of reflecting surfaces employed, I took five mirrors, and found that on exposing them to the sun I obtained with only one, less heat and light than if I used direct sunlight. With two the light and heat increased considerably; three gave as much heat as an ordinary fire, and four gave me a still greater effect. I therefore concluded that by multiplying these plane mirrors, I not only obtained greater effects than those got