From eyeglass to kijkglas: A history of the telescope

By Ian More

The majority of sciences depend on observation as one of their major investigative tools. This is peculiarly the case with astronomy which is perhaps the oldest recorded science with observational records from ancient Babylonia, China, Egypt and Mexico. The first true astronomers were the Greeks who deduced that the earth must be a sphere and measured its size with some accuracy (Eratosthenes of Cyrene). Star catalogues were drawn up by Hipparchus and the Almagest by Ptolemy of Alexandria summarized Greek astronomy. No visual aids existed at the times of the ancient Egyptians, Greeks or Romans and all the information was on the basis of naked eye observation.

Work was however carried out on the nature of light in early times and Euclid wrote about its reflection and refraction. Seneca born in 4 BC is reputed to have read all the books in Rome using a globe filled with water as an aid to vision. The effects of fragments of glass in focusing the suns rays were known and Aristophanes refers to the use of 'burning glasses' to burn holes in parchments and to erase writing on wax tablets. No formal aids to vision were available however and a prominent Roman in 100 BC stressed his resignation to old age and complained that he could no longer read himself but had to rely on his slaves.

Whilst Phoenicians cooking on sand discovered glass around 3500 BC it was around 1000 AD before the reading stone or magnifying glass was used ' Presbyopic monks used a segment of a glass sphere laid against reading material to magnify letters. The oldest known lens was found in Nineveh and was made of polished rock crystal one and a half inches in diameter.

Glass of reasonable quality became available by end of the 13th century. Grinding and polishing of glass was established in the great centers of Venice and Florence. The Venetians produced glass for 'reading stones' and mounted them in a frame in front of the eyes instead of directly on the reading material however these large magnifying glasses were very cumbersome and it was only when Venetians made small disks of glass that they could be fitted into a small frame directly in front of the eyes. Because these little disks were shaped like lentils they became known as 'lentils of glass' or lenses from the Latin.

It is perhaps surprising that a telescope was not invented around this time but it may be that church pressure militated against such a finding ' thus such objects were initially regarded as evil and contrary to the teachings of the church - their users or proponents were hounded by the inquisition. In 1268 Roger Bacon, a Franciscan monk, wrote in his Opus Majus 'if anyone examines letters or other minute objects through the medium of crystal or glass or other transparent substance, if it be shaped like the lesser segment of a sphere with the convex side toward the eye, he will see the letters far better and they will seem larger to him. For this reason such an instrument is useful to all persons and to those with weak eyes.' The latter was acceptable however he also wrote that (by their use) ''the Sun and Moon and stars may be made to descend hither in appearance' and for this he was labeled a magician and imprisoned. At a time when punishment for 'heresy' was extreme the description of findings contrary to church teaching was a risky business and might therefore be suppressed or altered in the interest of self preservation. The use of lenses to aid the eyesight was a much safer proposition. In 1289 Popozo wrote in a manuscript entitled Trait de con uite de la famille 'I am so debilitated by age that without the glasses known as spectacles I would no longer be able to read or write. These have recently been invented for the benefit of people whose sight has become weak'. Thus spectacles appeared between 1268 and 1289 ' the name of their inventor remains obscure although the name of Salvino degli Armati, a contemporary of Roger Bacon has been mentioned as a likely candidate. It is important to note that lens development and the construction of the telescope were the province of craftsmen and not of scientists. Since the majority of craftsmen of the day were illiterate their work and thoughts are not documented and hence not available to us in historical documents today.

The earliest illustrations of spectacles date from about 1350 and spectacles soon came to be symbols of learning. The artist Tommaso da Modena did a series of frescos in 1352 depicting brothers busily reading or copying manuscripts: one holds a magnifying glass whilst another has glasses perched on his nose. Once the precedent had been established the placing of spectacles on noses became popular probably as it denoted wisdom and respect.

The first spectacles had quartz lenses because glass lenses had not been developed. The lenses were set in bone, metal or leather settings and riveted together in an inverted V shape so that they could be perched on the nose. One of the major problems for the next 350 years was how to keep them on! Loops, strings, counter weights were all tried but it was not until 1730 that Edward Scarlett, a London optician, made rigid sidepieces, which sat on the ears. The problems are still with us to the present day as anyone who is breaking in a new pair of spectacles will testify! Interestingly tinted glasses are not a recent innovation! Lenses were made of a variety of different colors with turquoise, green and yellow being recommended but not amber or red!

It was a spectacle maker who probably assembled the first telescope. Hans Lippershey (c1570 ' c1619) a German born Dutch lens maker, is often credited with the invention but it is likely that he was one of many. The story is told that he was handling lenses in his shop when he happened to look through two differently shaped ones at the same time. He was holding them towards the doorway and was startled to see a distant church tower seem to jump to the door of his shop. He was amazed to see even the weather vane clearly. He initially used the phenomenon to attract shoppers by setting up two lenses to view the church. He eventually mounted the lenses one concave and one convex, in a tube to make what he called a kijkglas or "look glass" and it had a magnification of some 3 to 4 times! He applied for a patent intending it for use as a military device, but it was denied because 'too many people have knowledge of this invention'! This was true in that at that time telescopes were already for sale in Paris and other spectacle makers were claiming that they had invented the telescope! Whatever the truth it was news of Lippershey's invention, which reached Galileo Galilei (1564 - 1642) who was then Professor of Mathematics in Padua, Italy and stimulated him to make his own.

Galileo's version of Lippershey's far looker was not invented specifically for sky gazing ' the military significance of such an invention was not lost on both men. Galileo made firstly a X3 instrument and in August 1609 an 8X instrument which he presented to his ruler, the doge Leonardo Dona of Venice. It was inevitable that he would point it at the skies and with a 20X instrument he described craggy features of the moon, a vastly increased number of stars in the sky and 4 moons circulating around Jupiter. He published Sidereus Nuncius/The starry messenger in March 1610 and the book was a sensation throughout Europe. These incredible finds rocked the world because at that time everything was thought to circulate around the earth! Prior to that time no one could see clearly what was in the skies and much of what was believed about the universe was based on religious teaching with which he now found himself in conflict! His observations refuted the ideas of Aristotle taught at church run universities because they made possible for the first time the heliocentric ideas of Polish astronomer Nicolaus Copernicus (1473 - 1543). A Polish canon, Copernicus began to make observations around 1497 (although he relied mainly on the data of others) and demonstrated that the Sun and not the Earth was the centre of our planetary system. Considerable opposition existed however for removing the Earth from its central position in the universe and the Catholic Church was openly hostile. His work was published in De revolutionibus orbium coelestium/on the revolutions of the heavenly spheres its publication being overseen by a Lutheran minister. The latter added a preface without Copernicus's permission, which stated that the theory was intended as an aid to the calculation of planetary positions and not as a statement of reality. While this compromised the value of the text in the eyes of many astronomers it also saved the book from instant condemnation by the Roman Catholic Church. De revolutionibus was not placed on the index of forbidden books until 1616 and was removed from the list in 1835. Galileo's persuasive Dialogo sopra I due massimi sisterni del mondo/Dialogues on the two chief systems of the world 1632 was banned by the church authorities in Rome and he was made to recant by the inquisition. He was tried for heresy in 1633 and put under house arrest for his last years. Initially verifying Galileo's work was difficult because no one had telescopes of sufficient quality to see the moons of Jupiter. His lead was therefore one of craftsmanship not theory and by six months later adequate telescopes were in use to confirm his work. The next major discovery, that of sunspots was made by Galileo and others simultaneously. His initial telescope consisted of a plano-convex objective with a focal length of 30 to 40 inches and a plano-concave ocular with a focal length of 2 inches. The ocular was in a little tube, which could be adjusted for focusing. The objective lens was stopped down to an aperture of 0.5 to 1 inch and the field of view was about 15 arc minutes (i.e. 15 inches in 100 yards). The instruments magnification was 15 to 20X. The lens glass had a greenish tinge because of its high iron content and was full of little bubbles. The shape of the lens was good in its centre but poor at the periphery hence the use of a small aperture. The main problem of the telescope was therefore its small field of view and although higher magnifications were made they rapidly became useless because of this restriction.

From studies on the structure of the human eye Johannes Kepler (1571 - 1630) showed that a telescope could also be made with a convex objective and a convex ocular. The image was inverted but was much brighter and there was a larger field of view. Since this type of image does not present a major problem in astronomy the idea of an astronomical telescope for the astronomical community and a terrestrial telescope for more general use in which a third convex lens was used making the image erect but less bright.

The first telescopes allowed signifiicant magnification but produced fuzzy images - ie there was empty magnification - magnification with insufficient resolution of detail. This was due to a variety of problems but two were of particular theoretical importance. In a thin spherical lens not all rays from infinity ' incident parallel to the optical axis ' are united at one point. Those farther from the optical axis come to a focus closer to the back of the lens ' so called spherical aberration. In order to eliminate this Rene Descartes in his Discourse on Method of 1637 showed that lens curvature had to be either plano-hyperboloidal or spherico-ellipsoidal. This demonstration led to many attempts to construct such lenses but the technology to grind them was not available at that time.

The other major problem was that when light is passed through a prism it is split into its primary colors just as when light is passed through raindrops to produce a rainbow. This happens to a variable extent when light is passed through a lens and the resulting different colored images do not register exactly with one another. Thus stars seemed to be surrounded by rings of color. Isaac Newton ( 1642 - 1727) laid the basis for this phenomenon in his paper on light and color in 1672 and later in his book Optik of 1704. He showed that white light is a mixture of colored light with differing degrees of refraction. Therefore if light is passed through a curved lens the light is decomposed into the colors of the spectrum each of which then comes to a focus at a different point on the optical axis ' so called chromatic aberration. Newton was pessimistic that these problems could be solved and he therefore turned to the use of mirrors instead of lenses, thereby bypassing the problem - this resulted in the reflecting telescope, a great advance in clearer viewing. J Gregory (1638 ' 1675), a Scottish mathematician, had published a description of a reflecting telescope in Optica Promota in 1663 but he never actually constructed a telescope himself and it was left to Newton to put theory into practice. In 1668 he cast a 2 inch mirror blank of speculum metal and ground it into a spherical shape. This was placed in the bottom of a tube and the reflected rays caught on a secondary mirror angled at 45 degrees, reflecting the beam onto a convex ocular lens outside the tube. It was some 50 years later before the problem of chromatic aberration was solved in refracting telescopes.

Reflectors also had their problems. Others were unable to grind mirrors as Newton had done and Newton's mirror, which was made of bell metal, copper, tin and a little arsenic, was a combination, which dulled very quickly and had to be resurfaced ' a difficult and expensive process. Two centuries later Foucault discovered how to layer silver on glass, a process used until aluminum layering was developed.

In the original Newtonian reflector a small mirror, placed at an angle reflected the light from the main mirror to one side where it was viewed by an eyepiece etc. In 1672 a French sculptor named Sieur Guillaume Cassegrain placed the second small mirror such that it reflected the light directly back down the tube and through a small hole in the primary mirror to the eyepiece. The idea was ridiculed by Newton and such was his pre-eminence at that time that Cassegrain's model was initially overlooked. The present day Hubble Space Telescope is of course of Cassegrain design.

Although reflectors were better than refractors there were many more people making lenses than mirrors and hence much time and effort went into improving refractors. Refractors were easier to get and revealed a larger area of the sky. The discovery that increased magnification was a function of the length of the telescope tube and that brightness increased with lens size led to the construction of bigger and longer telescopes. Eventually the really long telescopes required scaffolding and cranes to hold them up and were difficult to maneuver. Thus mechanical stability became the new limiting factor. Nonetheless some of these high magnification telescopes were enormous. Christian Huygens (1629 ' 1665) and his brother Constantine constructed scopes, which were 12, 23 and even 123 feet long, whilst Jean Dominique Cassini (1625 - 1712) who discovered some of the moons of Saturn and the eponymous division in its ring system used scopes of 17, 34, 100 and 135 feet long. Every time he constructed a larger scope he discovered another moon! Another astronomer made telescopes of 300 and 600 feet long and he planned to build one of 1000 feet. Such long telescopes were useless for observation: it was almost impossible to keep the lenses aligned and any significant wind would make the instrument vibrate wildly. After about 1675 astronomers did away with the tube ' the objective lens was mounted on a building or pole and aimed by means of a string: the image was found by trial and error: the compound eyepiece (field lens and ocular) was placed on a little stand to receive the objective image ' such telescopes were called aerial telescopes.

It was around this time that the problems of chromatic aberration were solved in refractors. It was discovered that by putting one flint concave lens glass against a crown glass convex lens that the two lenses broke up the light at different angles and that they could in effect be made to cancel out each others errors. This was happened upon by chance by Chester Moor Hall (1703 ' 1771) and was utilized by the telescope maker John Dollard. This combined with a method of stirring glass as it was forming by Pierre Louis Guinand meant that lens glass was much more free of defects e.g. bubbles and hence the construction of larger and larger lenses became feasible.

It was in the first few decades of the 18th century that reflecting telescopes became practical - firstly in the hands of James Hadley and then by James Short and William Herschel. William Herschel (1738 ' 1822) was a Hanoverian musician, who settled in England and who was obsessed with the development of reflectors. He liked the clearer image of the reflectors and decided to build 5 or 6-foot models. No mirrors were available for this size of scope and he had to make and polish the required mirrors himself. He made seven, 10 and 20 feet models and finally a 40-foot model. He was able to see farther into space than anyone at that time and discovered Uranus, double stars, nebula and was the first to suggest that there may be other galaxies like our own. He discovered several new moons around Saturn. In 1846 Johann Galle, following calculations by British astronomer John Couch Adams and French astronomer Urbain Jean Joseph Leverrier, located Neptune. By the 1860s good photographs of the moon had been obtained and by the end of the century photographic methods had started to play a leading role in research

The introduction of computers to process the number crunching implicit in complex lens design aligned with computer control of the lens grinding and polishing system revolutionized the production of mirrors and lenses and led to the construction of larger and larger telescopes. The building of suitable structures to house these new instruments now became important and since stability of seeing was seen to be critical the importance of the siting of such telescopes became a concern. Milestones in telescope production include the Oschin ( or big Schmidt) telescope, a 48 inch combined refractor and reflector, the 200-inch Hale at Palomar and the twin Keck telescopes (each 8 stories high and weighing 300 tons) on Mauna Kea in Hawaii. This has progressed to the present day where telescopes on tops of mountains etc have been superceded by their placement in space with the use of rockets, satellites, space stations and space probes. When the US launched the Hubble Space Telescope in 1990 it was the most powerful optical telescope yet constructed, with a 2.4m/94.5 inch mirror. It detects celestial phenomena 7 times more distant than any land telescope - its replacement is already under construction. Whilst the telescope was conceived to enable scientists to better study the visible spectrum it is now used to exploit the whole electromagnetic spectrum from radio waves to gamma rays and this together with absorption spectrophotometry is therefore able to give us not only topographical information on the universe but also detailed insights into its chemical and physical make up and the time course of its evolution by its ability to delve further back into deep space.