Galileo's Glassworks: The Telescope and the Mirror

The invention of the telescope in late 1608 transformed Galileo's life and the future course of astronomy. News of a simple instrument that could magnify distant objects had reached The Hague at a time when the Dutch Republic and the Spanish Netherlands were locked in bitter conflict. Both sides quickly saw the military potential of the novel spyglass, but the early examples were disappointing, the magnification being only threefold. Within months, however, the Dutch telescope was available on sale in Paris.

Eileen Reeves considers the lapse of time between the creation of the telescope and Galileo's application of it for astronomical observation. Her investigation suggests that Galileo initially misunderstood the nature of the invention, wrongly assuming that it incorporated a concave mirror. Two lens-and-mirror designs were already under scrutiny, but both were unsatisfactory.

Interest in telescopic devices based on the concave mirror may have emerged in antiquity. Medieval legends describe wondrous mirrors that were available to the ancients but were long lost, rusted or broken by the Middle Ages. Virgil had fashioned a mirror to protect Rome by dazzling the enemy at the gates. The Pharos of Alexandria was imagined to have an imperial mirror capable of immolating ships from Byzantium or, in an alternative version, able to see approaching ships that had books suitable for the Library.

Writing in about 1250, the Franciscan natural philosopher Roger Bacon described mirrors and lenses capable of making stars "shine in what place you please". He included the anecdote that Julius Caesar's great glasses allowed him to spy on the castles of Britain from the coast of Gaul.

In the century before the telescope, optical knowledge in Italy was already extensive. A treatise on refraction from 1538 alludes to the magnifying properties of two superimposed eyeglass lenses. Galileo himself taught optics from a practical point of view and took a strong interest in the parabolic concave mirror. A skilled instrument maker, he invented the military compass for finding the height of a distant object.

When did news of the Dutch telescope reach Galileo in Venice? From a study of the correspondence between Galileo and his contemporaries, Reeves sets the date as early summer 1609. With a rudimentary sketch to hand, Galileo now acted with alacrity, making his own telescope in short order. This prototype was clearly superior in terms of cost and utility to any combination of a lens and a mirror. He boldly approached the Doge of Venice with a new device capable of great use in military situations, claiming it as a fruit of 17 years of science at the University of Padua.

Galileo quickly improved the magnification from the eightfold ability of the instrument shown to the Doge so that within three months he had achieved a power of 20 times, at which point he made his greatest discoveries. His discovery of four large satellites of Jupiter, and the phases exhibited by Venus, provided the empirical support for the Copernican model of the solar system. Galileo's telescope resolved the nebulous Milky Way into a vast system of stars, thereby founding a new science of stellar astronomy.

Reeves's splendid account is a cultural and social history that sets Galileo's telescope in the rich landscape of optical science from the Middle Ages to the modern period.