Page:Popular Science Monthly Volume 70.djvu/431

Rh might have added with equal truth that when a man appears in a world wholly unprepared to comprehend him, not only are his thoughts neglected, hut his discoveries forgotten. The story that Archimedes set the ships of the Romans on fire by means of burning-glasses is not found in any author who lived near his time. Moreover, the captains of the vessels would hardly be so obliging as to hold their vessels stationary in order that the old philosopher might work his will on them. Yet the marvelous feats he accomplished on the same occasion and vouched for by credible witnesses are scarcely less incredible. It may be accepted as certain that Archimedes produced wonderful effects by means of his lenses, whether they were made of glass or of some other material. That the ancients as late as the age of Plutarch knew nothing of spectacles is clear from the negative testimony of this writer, whose works might be superscribed 'Concerning all Things and Some Others.' In one of his table talks he tries to explain why old people, when reading, hold the book at some distance from the eyes. He finds the reason to lie in Plato's theory of vision, which he also holds. This philosopher maintained, in common with almost all the thinkers of antiquity, that sight is produced by a sort of fluid substance passing from the visible object to the eye, somewhat in the shape of a cone, the eye being the apex. When the organ becomes weakened by age this attenuated substance is too intense to permit normal vision; so in order to weaken it the object must be held farther away. He finds a confirmation of this theory in the habits of those animals that seek their prey by night when their sight is most acute. The fluid emanating from the object is too strong to be properly commingled with the power of vision, as he expresses it, possessed by these animals, but is so weakened and diluted by the surrounding darkness as to enable them to see at their best. This may seem to us very puerile; it ceases to be so when we remember that to this day no one has been able to answer the question. How do we see?

Though the art of making glass of certain kinds is very old, spectacles had to wait on the discovery or invention of some method that would produce it perfectly transparent. Specimens of glass have been found in the Egyptian tombs that are more than four thousand years old, and glass bottles are represented on tombs at least fifteen hundred years earlier. In Mesopotamia the art of making glass has been traced for at least two thousand years B. C. But all the glass of antiquity was of inferior quality and was almost useless for purposes where the rays of light were to be transmitted unbroken and with undiminished energy. Mirrors were also made in Egypt thousands of years before the christian era. The materials used were obsidian, metal, zinc and silver. Glass mirrors are mentioned by Pliny, but as they were neither perfectly plane nor foliated they gave back a very imperfect image and were not much esteemed. The word translated