September General Meeting Report
Guest Speaker: Dr. Fred Watson, Orator, Author, Astronomer and Director of the Anglo-Australian Observatory, on “Birth and Early History of the Telescope” (or Spectacle Makers behaving badly).
The bits for making a rudimentary telescope first came together in the year 1608. So the typical textbook entry tells us. In 1609 the idea came to Galileo, he improved it, looked at the night sky, wrote a 24page book called “Sidereus Nuncius” and upset the Holy Order of Christianity by insisting that the Earth moves. Or so the story goes.
Not so fast, says Dr. Fred Watson. In 1608 the time may have been right for the idea of the telescope to emerge into public consciousness, but it was a long time in coming; 600 years in fact. Magnification, the seminal feature of telescopes, was known as far back as 1000AD, when a glass sphere called a reading stone was laid on top of the material to be read, magnifying the letters. During the Islamic Golden Age in the 10th and 11th centuries, Ibn Sahl and Ibn al-Haytham (Alhazen) made advances in the physical and mathematical understanding of optics that were essential to the development of spectacle quality lenses. Although the magnifying and diminishing properties of convex and concave objects was known in Antiquity (recall the ship-burning mirrors of Archimedes), lenses as we know them were introduced in the West only at the end of the thirteenth century. Glass of reasonable quality had become relatively cheap and in the major glass-making centres of Venice and Florence techniques for grinding and polishing glass had reached a high state of development.
Now at last one of the perennial problems faced by aging scholars could be solved. With age, the eye progressively loses its power to accommodate, that is, to change its focus from faraway objects to nearby ones. This condition, known as presbyopia, becomes noticeable for most people in their forties when they can no longer focus on letters held at a comfortable distance from the eye. Magnifying glasses became common in the thirteenth century, but these are cumbersome, especially when one is writing. Craftsmen in Venice began making small disks of glass, convex on both sides, that could be worn in a frame – spectacles. Because these little disks were shaped like lentils, they became known as "lentils of glass," or (from the Latin) lenses. Salvino D'Armate, an Italian, is credited with inventing the first wearable eyeglasses around 1284, and by the early 1400s eyeglass makers flourished throughout Europe. The earliest illustrations of spectacles date from about 1350, when spectacles came to be symbols of learning.
By about 1450 all the ingredients for making a telescope were there. There is some documentary evidence (but no surviving designs or physical evidence) that the principles of telescopes were known to Leonard Digges, Taqi al-Din and Giambattista della Porta in the late 16th century. However, the earliest known working telescopes were those that appeared in the Netherlands in 1608. Their development is credited to three individuals, Hans Lippershey and Zacharias Janssen, who were spectacle makers in Middelburg, and Jacob (James) Metius of Alkmaar. A lovely story tells how one of Lippershey’s young apprentices accidentally held up two spectacle lenses in line before his eye and suddenly saw birds sitting on a distant church-steeple come into focus. It was to the many patent disputes between these three individuals that Watson refers to with the subtitle of his talk. Like with most scientific advances, the telescope was initially built for the military. Even Galileo first believed the telescope would best serve as an instrument of war: "I have many and most admirable devices; but they could only be put to work by princes because it is they who are able to carry on wars, build and defend fortresses, and for their regal sport [will] make most splendid expenditures." An awareness that these devices held the key to understanding the cosmos would not arrive for another two hundred years. Anyway, the instrument was not called a telescope then. They were variously referred to as lookers, Dutch trunks, perspectives and cylinders, and sold in fashionable shops in Paris, London, and Heidelberg for hunters and sailors.
Galileo named his most successfully built looker "Old Discoverer" and commented in letters to his friends that it was the best of more than one hundred previous models he had built. Barely three feet long and containing two magnifying lenses, each one inch in diameter, it was capable of magnifying objects twenty-eight times their actual size. At a banquet held in Galileo’s honour on the night of April 14 in 1611 a visiting scientist, Giovanni Demisiani a Greek mathematician, peered through Old Discoverer and proposed that the refracting looker be given the name telescope, a Greek word meaning "to see into the distance”, and the name stuck. The telescope became one of the central instruments of what has been called the Scientific Revolution of the seventeenth century.
Galileo’s outspoken support for the Copernican theory that Earth revolves around the sun created the well-known friction between Europe's most famous scientist and Pope Urban VIII. In 1616 the pope ordered Galileo to admit he had erred, to admit that his “looker” did nothing more than create optical illusions. Following years of charges and countercharges, Galileo was in 1633 tried by a church court, found guilty and placed under house arrest for the duration of his natural life. But by then the evolution of the telescope was well on its way and could not be stopped anymore.
Endeavouring to side-step the troublesome chromatic aberration of the early telescope lenses Newton in 1668 came up with a design for a reflecting telescope, a telescope without lenses based on James Gregory’s information of 1663 that still bears his name. Then the Frenchman Guillaume Cassegrain, in reviewing Newton's telescope in 1672, found a solution to the annoying features associated with Newton’s tilted secondary mirror and side-mounted eyepiece. His design (named after him) was to have the secondary mirror reflect the collected light straight back through a hole in the main mirror to a rear-mounted eyepiece. Most modern reflector telescopes are now built this way. But reflecting telescopes at the time had problems of their own with poor quality tarnishing speculum metal (three parts copper, one part tin and one part arsenic) mirrors. Newton’s own telescope was reportedly only 15% efficient. Seventeenth-century astronomers found themselves spending more time grinding and polishing instead of stargazing. The problem was alleviated only with the introduction of silver coated mirrors in 1857 and then aluminized mirrors in 1932. There appears to be no limit in sight for the size of reflecting telescopes, with several 10 metre behemoths already in operation. With the creation of the first multi-element achromatic lense by George Bass in 1733 (patented by Dollond around 1750) refracting telescope lenses too gradually increased to a maximum physically practical size of 1.02 metre (40 inches) in the Alvan Clark refractor at Yerkes Observatory, Wisconsin USA in 1897.
Accompanied by many illuminating slides and many more humorous anecdotes Dr Fred Watson thus reflected and refracted on a brief History of the Early Telescope. Another fascinating episode in the lead-up to the International Year of Astronomy, the 400th anniversary of the birth of the telescope. . ❑
Alfred Klink