Invention Of The Telescope

Claims of prior invention

In 1682, the minutes of the Royal Society in London Robert Hooke noted Thomas Digges' 1571 Pantometria, (a book on measurement, partially based on his father Leonard Digges' notes and observations) seemed to support an English claim to the invention of the telescope, describing Leonard as having a fare seeing glass in the mid-1500s based on an idea by Roger Bacon. Thomas described it as "by proportional Glasses duly situate in convenient angles, not only discovered things far off, read letters, numbered pieces of money with the very coin and superscription thereof, cast by some of his friends of purpose upon downs in open fields, but also seven miles off declared what hath been done at that instant in private places." Comments on the use of proportional or "perspective glass" are also made in the writings of John Dee (1575) and William Bourne (1585). Bourne was asked in 1580 to investigate the Diggs device by Queen Elizabeth I's chief advisor Lord Burghley. Bourne's is the best description of it, and from his writing it seemed to consist of peering into a large curved mirror that reflected the image produced by a large lens. The idea of an "Elizabethan Telescope" has been expanded over the years, including astronomer and historian Colin Ronan concluding in the 1990s that this reflecting/refracting telescope was built by Leonard Digges between 1540 and 1559. This "backwards" reflecting telescope would have been unwieldy; it needed very large mirrors and lens to work; the observer had to stand backwards to look at an upside down view, and Bourne noted that it had a very narrow field of view, making it unsuitable for military purposes. The optical performance required to see the details of coins lying about in fields, or private activities seven miles away, seems to be far beyond the technology of the time, and it may be that the "perspective glass" being described was a far simpler idea, originating with Bacon, of using a single lens held in front of the eye to magnify a distant view.