Did the reflecting telescope have English origins?

Optics Group help with research

The photograph below, taken on the roof of the Physics Department, appeared in The Times of 18 August, illustrating an article by the newspaper's Technology Correspondent on the replica of an Elizabethan telescope which was recently built in the physics Department by Colin Ronan, seen testing the device.

The crude telescope would have enabled English commanders to sight Spanish galleons, and given Elizabethan astronomers unrivalled glimpses of the moon and stars. It was the subject of the BBC television programme, "The Sky at Night", broadcast on 16 August.

Colin Ronan is an historian of science and author of several works on astronomy. He was President of the British Astronomical Association in 1989-91. In his second presidential address lay years, he put forward the claim that the credit for the invention of the reflecting telescope should go to Leonard Digges, an English mathematician and surveyor, whose device predates rival Dutch claims of 1608 by over thirty years.

After giving a talk on the subject to the Optics Section of the Physics Department last May, Mr Ronan was invited by Professor Dainty to use the facilities of the section to construct a replica of Digges's telescope.

He was assisted in his work by a member of the Optics section, Mr Gilbert Satterthwaite FRAS (right), who runs the teaching lab for the MSc course in Applied Optics. Mr Satterthwaite also lectures in Astronomy for the University of London Centre for Extra-Mural Studies at Birkbeck College, and at the Greenwich Planetarium. He is the author of an Encyclopaedia of Astronomy (1970) and editor of the 1973 revision of Norton's Star Atlas.

The design of the telescope is based on a description given in a manuscript report on military and naval inventions, written in 1578 by the mathematician William Bourne, for Lord Burghley, Elizabeth I's Secretary of State.

According to the report in The Times, 'Building the reflecting telescope, which consists of a convex lens at the front and a curved mirror mounted at the back, led to some surprising findings. Mr Ronan discovered that when images are viewed from below or from the side they are inverted, which Mr Ronan says, for military and naval purposes "would have been a bit of a devil".

What solved this difficulty was referring to work by Sir Isaac Newton of a century later. Newton designed a reflecting telescope which also gives an inverted image yet his drawings of a weathercock are portrayed upright.

The solution to seeing an upright image, and which is how the Elizabethan telescope has been built, requires a viewer to stand over the end looking backwards into the box.

This version of the Digges telescope, which magnifies images 11 times, has a small field of view of about 0.4 of a degree but this would have been very close to that of Galileo's," said Mr Ronan.

However he stresses that the magnification would have "been damned useful at picking out a ship on the horizon and seeing if it was friend or foe".

He is convinced that not only did the Digges family build the reflecting telescope but also probably experimented with refracting telescopes of the kind which Dutch rivals sold in 1608 and which Galileo used in 1609.

Unfortunately, Nick Nuttall's article was accompanied by the reproduction of a drawing of what is described as 'Galileo's later telescope'. Gilbert Satterthwaite points out for the record that this is not Galileo's telescope but the helioscope of C. Scheiner, circa 1630.

Colin Ronan's Presidential Address in which the case for Digges is developed was published in the Journal of the British Astronomical Association, 101, 6, 1991.


Leonard and Thomas Digges

Colin A Ronan, M.Sc., F.R.A.S.

By enabling man to explore the Universe far beyond the range of the naked eye, the telescope is one of the most powerful of all scientific instruments. According to tradition, it was invented in Holland around 1608. There is evidence, however, that it originated more than thirty years earlier, in England, and that the inventors were Leonard and Thomas Digges. There is reason to suppose that it was the latter, and not Galileo, who first turned the telescope to the night sky, observing myriads of stars invisible to the naked eye. He concluded that the Universe was infinite in extent.

The date of birth of Leonard Digges remains uncertain; some time in 1520 is the accepted figure. A member of the landed gentry of Kent, who studied at University College, Oxford, he never took his degree though 'his ample means and leisure were devoted to scientific pursuits' (1). An able applied mathematician, he soon gained an enviable reputation in surveying and also as an author. In 1553 he produced A General Prognostication, to be followed two years later by an enlarged edition called A Prognostication of Right Good Effect and still another revision the next year, 1556, with the title Prognostication everlasting. In the hands of his son, this was to become a seminal work.

Written in English rather than Latin, the lingua franca of the learned world, the book contributed greatly to a wider dissemination of the principles of astronomy, especially to those artisans and others to whom Latin was unfamiliar, for these 'Prognostications' contained a wealth of information. There were useful rules and tables for astronomy and astrology (which was not then separated from astronomy as it was later to be); calendar dates of moveable feasts for several years to come; tables of the Moon's motion; a description of how to tell the time during day or night; descriptions of meteorological phenomena and an account of their causes; and even tables for bloodletting, computed - as was then customary - for propitious astrological times. The book also contained a short account of the Universe according to the traditional system of Ptolemy with tables of the dimensions of the planets and their orbits.

Prognostication was a best-seller and much enhanced Leonard Digges's reputation. In addition he published, also in 1556 and in the vernacular, a practical book about land-surveying. With the title Tectonicon it concerned 'the exact measuring and speedie reckoning (of) all manner of Land, Squares, Timber, Stone, etc. Further, declaring perfect making and large use of the Carpenter's Ruler, containing a Quadrant geometrical; comprehending also the rare use of the Square (1). Another unpublished manuscript - Pantometria - was to be published posthumously, as we shall see.

One of Leonard Digges's close friends was the scholarly Dr John Dee whom we shall meet again presently. Dee was a collector of both books and manuscripts, and his private library at Mortlake in Surrey contained many texts by Roger Bacon. It was no doubt here, on his visits from his home at Wotton in Kent, that Leonard came across Bacon's references in his Opus Majus to lenses and the ability to use them to '.....cause the sun, moon and stars in appearance to descend here below......' (2) Stimulated by Bacon's work, and perhaps by other texts in the library, Leonard set about to determine at least the principles of refracting and reflecting telescopes and, as will become evident, to construct at least a reflector. For Leonard Digges not only had a theoretical mind but also an eminently practical flair as well. He was, indeed, an able experimental scientist at a time when what is now often referred to as the Scientific Revolution was still something very new.

The times in which Leonard Digges lived were deeply troubled. In England especially the struggle between Catholics and Protestants was growing in intensity. Mary Tudor ascended the throne in 1553, restored Roman Catholicism as the state religion, and the next years was to be married to Phillip II of Spain. This triggered a rebellion of the men of Kent under St Thomas Wyat, who considered such a marriage an affront to national honour. The rebellion failed and in April 1554, Wyat was executed. Leonard Digges, who had taken part in the insurrection, was also condemned to death and his property confiscated. However, he received a reprieve, possibly due to the efforts made by a kinsman, Lord Clinton, to whom he has dedicated his Prognostication, but Leonard has still to redeem his property. This took him until May 1558. Sadly, he did not survive for long afterwards, dying probably some time in 1559.

Dr John Dee

In spite of a character assassination - because of Dee's interest in mysticism and psychic research - started by Marc Casaubon fifty years after Dee's death and carried on until almost the second decade of this present century, that gossiping antiquarian John Aubrey could still refer to him as 'one of the ornaments of his Age' (3). Born in 1527 to a mercer in the City of London at a time when Henry VIII still ruled England, Dee was later educated at St John's College, Cambridge, graduating in 1545. Later he became a Fellow of St John's and a foundation fellow of Trinity. He visited Paris and Louvain, studying at Louvain University with Gemma Frisius and Gerardus Mercator and thus laying the foundations for his own notable contributions to geography.

Dee became a highly respected scholar both in England and in Western Europe, a welcome frequenter at Court, and adviser on the chief English voyages of discovery undertaken in his day. Yet though he wrote books about navigation, he did not publish them but kept them as manuscripts; few now survive. During the third quarter of the sixteenth century, Dee was 'the guiding spirit' (4) of the English school of mathematicians. He wrote a notable preface to the first edition in English of Euclid's Elements of Geometry, published in 1550, and translated by Henry Billingsley but clearly edited by Dee himself. In this he discusses the relationships and applications of mathematics, and also gives support to the new mathematical elegancies of the Copernican theory.

It was to Dee that Leonard Digges entrusted, in the event of his early death, the upbringing of his son Thomas. At a time when life expectancy was low, especially in view of the religious turmoil of the times, Leonard's move was a wise and far-sighted one.

Thomas Digges

Thomas Digges was bout thirteen when his father died, and with this 'second parent' in astronomy and mathematics as his new mentor, it is not surprising that Digges was to become famous in his own right. Noted as a surveyor as well as a mathematician, his first book appeared in 1571, when he was twenty-five years of age. Ostensibly , this was a publication of one of his father's works; its title was A Geometrical Practise, named Pantometria and to it we shall return presently. Two years later, in 1553, Thomas published his own Alae seu Scalae Mathematicae....., dedicated to Lord Burghley. Like Dee, Burghley had been up at St John's College Cambridge but was then biding his time after serving Edward VI as a junior secretary, having declined any post in Queen Mary's administration. Thomas's book concerned the scale of space and paid particular attention to the supernova of 1572, now closely associated with the name of the famous Danish astronomer Tycho Brahe. But Brahe was not alone in observing the phenomenon, and Thomas Digges made some particularly accurate measurements of it. Indeed, these had an error of less than two minutes of arc and were second only to those of Brahe himself, who se accuracy was a byword. For this reason, Brahe actually devoted over thirty pages to Digges's work in his chief book on the star.

Incidentally, in the same year Dee's own book on the parallax or distance of the supernova appeared, carrying a preface by his ward Thomas Digges. Except for the Alae, Thomas wrote in English, following his father and many contemporary English mathematical authors; a trend to be followed later by Galileo.

In 1576 Thomas Digges produced his father's Prognostication everlastinge but in an augmented edition. This has become an historic publication because Thomas appended to it a work of his own. A Perfit Description of the Caelestiall Orbes, in which he propounded the view that the Universe was infinite in extent. But more of this anon. Three years later, in 1579, he produced a book on military surveying and ballistics with the title An arithmeticall militare treatise name Stratioticos..... By the time this book appeared, Thomas had been sitting in Parliament as the member for Wallingford for the previous six years; later, in the Parliament of 1585, he represented Southampton. In 1582 he was appointed to a commission for considering the repair of Dover harbour, and when the work was actually started, he was the chief overseer. The next year, 1586, Thomas was appointed Muster-Master General to the English forces in the Netherlands, a post he held until 1593. And as if this were not enough, about 1590 Queen Elizabeth, who had ascended the throne a couple of years before, appointed Thomas as a member of a commission to fit out a fleet for the discovery of lands in '..... The antarctic seas and especially to the dominions of the great "Cam of Cathaia" [China]'. [1]

Thomas Digges was a good practical scientist who even experimented to refute the old physics of Aristotle in order to promote the Copernican theory. He died in August 1595 at the age of forty-nine, and was survived by two sons and two daughters. Whether his wife Agnes survived him is unclear, as in the precise date of his marriage to her. A monument to him was erected in St Mary Aldermanbury, but did not survive the Great Fire of London. [5]

The invention of the telescope

Figure 1

The title-page of Pantometria by Thomas Digges. This edition of 1571 contains a reference to 'Perspective glasses' and is in the possession of the Royal Astronomical Society.

Figure 2

The use of a plane mirror for determining the distance of ships at sea, from Digges' Pantometria of 1571.

Figure 3 (a and b)

Part of the manuscript of William Bourne's report about telescopes to Lord Burghley. (Manuscript Landsdowne 121, item 13.) By courtesy of the Trustees of the British Library.

Figure 4

The diagram by Thomas Digges of an infinite universe, which appears in an addendum to his edition of 1576 of his father's A Prognostication everlastinge......Courtesy of the Royal Astronomical Society.

It is now well known that claims for letters patent for a refracting telescope were laid before the States General of the Netherlands in October 1608. The first claimant was Hans Lipperhey (or Lippershey) but though instructed to keep the invention secret - for such a device could be of military importance to a country then fighting for its very survival - two other claims were made during the same month. Clearly, the invention was not as original as the States General first believed; indeed, a story became current a little later which told how the spectacle-maker Lipperhey had been visited by a fellow countryman and asked to prepare one convex and one concave lens. Later calling to collect them, he held them up together and demonstrated how in combination they could be made to enlarge objects some distance away. Moreover, the son of Sacharias Janssen, another of the claimants, later stated that his father already had a telescope of Italian manufacture, dated 1590, at the time Lipperhey first approached the States General. It seems then, as a number of scholars have now concluded, that the origin of the telescope lay outside Holland and well before 1608. One of the favoured places is England, for reasons which will now become evident.

To begin with the evidence comes from printed books. The first is john Dee's in his Preface to Billingsley's translation of Euclid of 1570. In this he proffers advice to military commanders, and in discussing obtaining evidence about enemy forces, writes:

'He may wonderfully helpe him selfe, by Perspective glasses. In which (I trust) our posterity will prove more skillfull and expert, and to greater purposes, than in these days, can (almost) be credited to be possible.'

Here the phrase 'Perspective glasses' is one used by both Thomas Digges and Dee for telescopes (the word 'telescope' having been invented only in the seventeenth century).

The next, and very significant quotations come from Thomas Digges' edition of his father's book Pantometria, the full title of which, as figure 1 shows, contains the words 'Perspective glasses'. Appearing the year after Dee's Preface, they are far more specific. In the first Thomas writes:

'..... my father by his continual pain-full practices [practical experiments], assisted with Demonstrations Mathematicall, was able and sundrie Times hath by proportionall Glasses duly situate in convenient angles, not onely discovered things farre off, read letters, numbered peeces of money with the very coyne and superscription thereof, cast by some of his freends of purpose uppon Downes in open fields, but also at seven miles declared what had been doon at that instant in private places.....'

Perhaps this sounds a little exaggerated, particularly seeing things happening seven miles away, but what Thomas Digges is saying here may well follow on from what Dee published, namely that one could see the movement of bodies of troops. This would indeed be possible at a distance of seven miles with a magnification of not more than eight times, as a recent test by the author shows.

The second and more important quotation which follows a description of measuring the distance of ships from the shore using a flat mirror (figure 2) - a 'plaine Glasse' - and runs:

'Thus much I though good to open concerning the effects of a plaine Glasse, very pleasant to practise, yea most exactly serving for the description of a plaine champion country. But marveilous are the conclusions that may be performed by Glasses concave and convex of Circulare and parabolicall formes, using for multiplication of beames sometime the aide of Glasses transparent, which by fraction [refraction] should unite or dissipate the images or figures presented by the reflection of the other. By these kinde of Glasses or rather frames of them, placed in due Angles, yee may not only set out before your eye the littely image of every Towne, Village, etc. and that in as little or great space or place as ye will prescribe, but also augment and dilate any parcell thereof, so that whereas at the first appearance an whole Towne shall present it selfe so small and compact together that ye shall not discerne any difference of streates, ye may by application of Glasses in due proportion cause any peculiare house, or roume thereof dilate and shew it selfe in as ample forme as the whole towne first appeared, so that ye shall discerne any trifle, or read any letter there lying open, especially if the sunne beames come unto it, as plainly as if you were corporally present, although it be distante from you as farre as eye can discrye: But of these conclusions I minde not here more to intreate, having at large in a volume by it selfe opened the miraculous effects of perspective glasses.'

This is all very significant. In the first place, Digges describes what must obviously be a reflecting telescope, while his description can also be taken to include combinations of lenses (transparent glasses) and thus hints at the refractor also. Incidentally, when in the Netherlands he probably met the father of Jacob Adriaenszoon, the third Dutch claimant to the invention.

Secondly, Thomas describes the effects of using different magnifications or, as we should put it today, eyepieces with different focal lengths and thus different magnifying power. Thirdly, this is no theoretical description. Higher magnifications dilate the image and more light is required to show useful detail. Hence the need for the 'sunne beames' - a requirement which comes from practical use of the instrument and not, in the 1570s, from any optical theory. Yet many historians of science have been chary of taking all this at its face value, though none have explained why Thomas Digges should fabricate such a story in a book devoted to eminently practical instructions for surveying. Indeed, such action would seem wholly out of character.

Fortunately we do not have to rely only on Thomas Digges. There is independent testimony. Sometime about 1580, Lord Burghley, then chief advisor to Elizabeth I, faced with an impending invasion of England by Spain - the Armada, in fact, was only eight years away - felt it necessary to get a report on whether there was any substance in the claims of John Dee and Thomas Digges, about a telescope. Burghley commissioned a certain William Bourne (d.1583) to look into the matter. Bourne was an expert in navigation and in gunnery with four books to his credit, and his report [6] is still extant (figure 3). It is n astonishing document, as well as being a first class factual report.

In its opening pages Bourne explains techniques for making lenses and parabolically curved mirrors then, finally, describes:

'The effects of what may bee done with these last two sortes of Glasses: The one concave with a foyle, uppon the hilly side [i.e. a concave mirror, having a reflecting foil on the back] and the other grounde and polished smoothe, the thickest in the myddle, and thinnest towards ye edges, or sydes' [a bi-convex lens].

Then, after bemoaning the fact that he himself has neither the time nor the money to experiment with a concave reflector, Bourne continues:

'.....there ys dyvers in this Lande, that can say and dothe knowe muche more, in these causes then I: and specially Mr Dee, and allso Mr Thomas Digges, for that by theyre Learninge, they have reade and from many moo [sic] auctors in those causes..... But notwithstanding upon the smalle prooffe and experyence ..... Yet I am assured that the Glasse that ys grounde, beeynge of very cleare stuffe, and of good largeness, and placed so that the beame dothe come thorowe, and so reseaved into a very large concave lookinge Glasse, that yt will shewe the thinge of marvelous largenes, in a manner uncredable to bee believed of the common people.

In Bourne's day, and for most of the next two centuries, the size of optical lenses and mirrors was denoted not by their diameter but by their focal length - the longer it was the more powerful (and usually the larger in diameter) they were. Here Bourne refers to the important fact that for a reflecting telescope to magnify, the concave mirror must be very much larger in focal length than that of the lens used as an eyepiece. Yet perhaps more significant still, is what he next says:

'Wherefore it is to be supposed, and allso, I am of that opinyon, that having divers and sundry sortes of these concave Looking Glasses, made of great largeness......yt ys lykely yt ys true to see a smalle tinge, of very greate distance, ffor that the one Glasse dothe rayse and inlarge, the beame of the other so wonderfully. So that those things that Mr Thomas Digges hathe written that his father hathe done, may bee accomplisshed very well, withowte any dowbte of the matter: But that the greatest impediment ys, that yow cannot beholde, and see, but the smaller quantity at a tyme.'

If one had any doubt about Bourne's report being no more than a report of previous claims and rumours, he dispels this by describing how one uses an eyepiece of given focal length with mirrors of increasing focal length to obtain greater magnifications and, above all, by his very last remark. This states that a telescope has only a small field of view. In the sixteenth century - and for probably a century and more later - this was something one could not calculate theoretically. Thus it is evident that Bourne had indeed observed through an actual telescope.

Epilogue

If Thomas Digges had a reflecting telescope, it might be expected that, with his interest in astronomy, he had tried using it on the heavens. If he did, this would prove still further confirmation. Such appears to be the case.

In his important addendum to his father's Prognostication everlastinge of 1576, he not only describes and supports the Copernican theory, but links it with an infinite universe. This revolutionary concept appears not only in the text, but also in a diagram (figure 4) and the legend above it is considerable importance. In this Thomas's reference to endless joy of the elect was probably taken over from the general belief that beyond the outermost sphere of space lay the 'abode of the blessed', while reference to the angels may have been influenced by Dee. Yet the rest of the description does not fit in with Dee's Hermetism - a widespread doctrine then current and mentioned by Copernicus. Indeed Thomas's text makes this clear, for he writes:

'Of which lightes Celestiall it is to bee thoughte that we onely beholde such as are in the inferioure partes of the same Orbe [i.e. the orb of the heavens], and as they are hygher, so seeme they of less and lesser quantitye, even tyll our sighte beinge not able farder to reache or conceyve, the greatest part rest by reason of their wonderful distance invisible unto us.' [7]

Now this is just what one would expect to see in the kind of reflecting telescope which Bourne describes, for since the reflecting surface was on the rear surface of the concave mirror (instead of the front surface as is current practice), ghost star images would appear, though noticeable only for the brighter stars. This surely is what a practical scientist like Thomas Digges would do; he had looked through a reflector at the night sky.

Thus it is that Leonard and Thomas Digges made notable contributions to astronomy. Leonard by his invention of an actual reflecting telescope, and probably the refractor also, Thomas not only by promoting it but also by his own use of it on the heavens and his linking of the Copernican system with an infinite Universe.

References

[1] 'The Compact edition of the Dictionary of National Biography' Oxford, University Press, Oxford, 1975, Vol. 1.

[2] Bacon, Roger 'Opus Majus', ca 1267. From 'The Opus Majus of Roger Bacon: a Translation by Robert Belle Burke', Philadelphia, 1928, 2, 582.

[3] See French, Peter J. 'John Dee', Routledge & Kegan Paul, London, 1972, and Yates, Frances A. 'Giordano Bruno and the Hermetic Tradition;, Routledge & Kegan Paul, London, 1964.

[4] See Johnson, Francis R. 'Astronomical Thought in Renaissance England', Johns Hopkins Press, Baltimore, 1937.

[5] From information supplied by Miss J. M. Wright of the Guildhall Library, and sent to me through the kindness of Mrs Norma Digges.

[6] M. S. Landsdowne 121, item 13.

[7] Digges, Thomas 'A Prognostication everlastinge......'. Thomas Mutch, London, 1576, 4 pages after N2. The text is in Gothic script with Roman upper and lower case for special words or phrases; I have inserted italics for such Roman type in the original to make a suitable distinction.


A 'Digges Telescope'

From Mr Colin Ronan and Mr Gilbert Satterthwaite

We understand that some members are interested in having details of the optical parameters of the 'Digges telescope' displayed on the 1992 August 16 transmission of The Sky at Night. We therefore present a diagram. Readers may also care to note that a similar instrument has been constructed at Leicester University since the broadcast. Like that of the instrument made by us, its field of view is very small, and confirms the remarks made by William Bourne, and quoted in the Presidential Address given in October 1991. [1]

[1] Ronan, C., J. Br. Astron. Assoc., 102, 339 (1191).


 


copyright Gilbert Satterthwaite 2002©
site designed by Concept Design Consultancy Limited