Telescopes (part 1)
400 years of tubes, objectives and eyepieces.
400 years of tubes, objectives and eyepieces.
As an optical instrument, the telescope is now a little over 400 years old. So next time you see a film with Columbus using one, you'll know the film director has boobed.
This web page concentrates on the telescopes, for siderial or terrestrial use, in our own collection or those illustrated in our pictorial collections. It also pays special regard to Dollond telescopes which were amongst the most popular in both Great Britain and abroad for a period of over one and half centuries.
Historians increasingly speak of a pre-history of the telescope. A 16th century play by Roger Green imagined Friar (Roger) Bacon using a viewing device set up in Oxford to discover an unfaithful couple's misdemeanours many miles away in Suffolk. Several similar references imply that by the Elizabethan period the conceptual idea of a perspective device, one that is looked through, was familiar, or at least understandable to an audience or readership. It became entwined with the certainly much earlier concept of the mirror of Providence.
In the beginning
The invention of the hand-held telescope is generally ascribed to the Dutch spectacle maker Hans Lipperhey of Middelburg who in 1608 discovered that holding a convex and a concave lens apart from each other, then looking through the two together, could produce a 'far-seeing' effect.
Not all historians agree and some have advanced the claims of Lipperhey's neighbour and rival, Sacharias Jansen, or Jacob Metius to have been the inventor. We could even be considering the wrong century since there is circumstantial evidence for an earlier refracting telescope designed by the English mathematician Leonard Digges and made by his son Thomas in the second half of the sixteenth century. This story, of a 'Tudor Telescope', is not generally, however, given much weight.
The more unreliable websites often cite Galileo as the inventor of the telescope. This is ridiculous. We know that he heard of Lipperhey's work in the Netherlands and this prompted him to begin his experiments. Galileo is correctly credited with having been the first to make extensive use of the refracting telescope for astronomical observations. Crucially he published his findings and those who read what he wrote, such as Thomas Harriott, say nothing in their own writings to indicate an awareness of any earlier instrument.
Lipperhey was actually born in Germany but moved to Middelburg in the Spanish Netherlands in 1594. In a famous incident of 1608 he arranged a demonstration of his new device to the authorities, with the intention of taking out a patent. Lipperhey is the man described as Un faiseur de lunettes de Mildebourg, pauvre homme, fort religieux & craignant Dieu (a spectacle maker from Middelburg, a humble man, very religious & pious) at the end of a report from a Siamese emissary. From the Maurits Tower in Den Haag they saw the clock of Delft and the windows of the church in Leiden, 'and even the stars'. As this was a time of truce, Spinola the commander of the Spanish army was present at the demonstration, a fact which may otherwise have raised eyebrows. Notwithstanding the mention of stars, which suggests that astronomical observation was a use for the telescope from the very outset, it was mainly conceived as having a military application.
In August 1609 Della Porta produced the first sketch of a telescope in a letter. We do not have any images of Lipperhey's instrument; indeed the first Dutch illustrations of telescopes do not occur until 1623-4. They were produced by Van de Venne who also happens to have lived in the same street as Lipperhey.
In 1610 Snellius wrote about a very small telescope in a letter to a friend. This is another very important early reference.
Did you know? The spelling of Lipperhey's name is often given incorrectly in the English literature as 'Lippershey' (with an 's'). The origin of this mistake has been traced back to a slip of the pen by Gerrit Moll, director of the Observatory at Utrecht, in a publication of his, dated 1831.
The Galilean telescope intercepted the convergent light rays from the objective end of the instrument before their arrival at the focal surface by means of a concave lens acting as an eyepiece, thereby forming an erect but virtual magnified image. Galilean refractors were relatively small in size. A typical objective lens for a later telescope of the same type might measure two inches.
Reconstructions have proved that you couldn't see much at all through a Galilean telescope. They were hard to use, the image was tiny and highly prone to vibration...even the act of breathing could affect its use.
Some original telescopes of Galileo are preserved in Florence. They are cylindrical, but it seems that other very early telescopes may have been trumpet-shaped. There is one dating from about 1630 at the Adler Planetarium in Chicago.
Although still in use today, in seaside viewing scopes, the inferior Galilean design was soon modified by the German astronomer and mathematician Johannes Kepler, who in 1611 replaced the concave lens of the eyepiece with a convex lens relocated behind the focal surface to form an inverted but real image, thereby also establishing the basic design of all subsequent eyepieces. Kepler's device offered more magnification and a wider field of view (useful when scanning the night sky for hard-to-spot celestial objects). Its upside-down image did not matter so much for astronomical observations, but the Keplerian telescope suffered from even worse chromatic aberration.
See elsewhere on this website for more on Kepler and astronomical optics.
It is a point not commonly appreciated that, after a burst of inventiveness from 1608-11, all possible discoveries relating to the design of telescopes were over by 1612 and telescopes thereafter were at best a curiosity for at least the remainder of the first half of the seventeenth century. The maximum magnification of which they were capable settled at about X30. Very large telescopes are a feature of the second half of the seventeenth century, when they could become very long indeed.
Not all telescopes relied solely on lenses. Some used metal mirrors to reflect the light from the objective end of the telescope to the observer's eye. The Scottish mathematician James Gregory published a design for a reflecting telescope in 1663, but the first usable instrument was built to a different design by Isaac Newton in or around 1670, apparently with knowledge of Gregory’s design. The Gregorian telescope proposed a concave primary mirror perforated by a central hole opposing a smaller concave secondary mirror located outside the focal point of the primary, such that light reflected from the primary would be captured by the secondary and reflected back through the hole in the primary to a focal surface. Newton modified this difficult-to-construct design by replacing the concave secondary with a flat mirror angled at 45 degrees to the optical axis of the primary mirror and by doing away with the central hole in the primary, such that light reflected from the primary would be intercepted by the secondary before coming to a focus, and directed at 90 degrees to a focal surface located out of the path of the incoming light. At about the same time as Newton and Gregory, and apparently working completely independently, a French priest Laurent Cassegrain is now believed to have been the inventor of a third basic design of reflecting telescope (the Cassegrain) similar to Gregory’s, but in which the secondary mirror is convex rather than concave and is instead located inside the focal point of the primary mirror. Although theoretically superior to both Gregorian and Newtonian designs due to the tendency of any error in the concave primary to be cancelled out by the convex secondary, the Cassegrain design did not become widespread until the late nineteenth century, because of the additional difficulty of making a convex mirror. In all designs, mirrors were made of an alloy of tin and copper with a small admixture of arsenic, also invented by Newton and called speculum metal. Although very brittle and hard to cast, a successful casting produces a highly reflective surface when polished.
The problem with reflectors was that the mirrors tended to oxidise very fast, at which point the telescope would become 'blind'. They were also difficult to manufacture since you couldn't really control the shape of the mirror during the production process. Reflectors do not distort the image they produce if you have a perfectly shaped mirror, but the difficulty of obtaining that perfection deterred many makers. Newton only ever made two.
Achromatic Refracting Telescopes
As Newton had observed, early optical devices suffered from colours at the edges of the image - 'chromatic aberration'. The dispersion of the light rays depends upon the quality of the refracting surface. Chester Moor Hall, a barrister who practised optical experiments as a hobby, worked to counter these 'deletrious chromatic fringes' and eventually solved it by commissioning a lens of two separate parts, made from different materials (one half concave flint glass, the other half convex crown glass) and with opposite powers. These cancelled each other out and produced an image which, whilst not perfect, went a good way towards the goal of interference-free optics.
In a fateful decision Hall commissioned one half from Edward Scarlett and one half from James Mann, but both opticians sub-contracted the project to the same lens grinder, George Bass. In 1747 John Dollond, then still an amateur optician, wrote to the eminent (and blind) Swiss mathematician, Leonhard Euler criticising his attempt to suggest that correcting spherical and chromatic aberration in object-glasses was possible. In the 1750s, by now an established working instrument maker, he began experimenting with objective lenses based on his interpretation of Newtons's Opticks.
Early efforts resulted in telescopes with multiple eyepiece lenses. Four of five of these lenses could combine to enhance the performance of a refracting telescope considerably. Refractors had to be very long, however, in order to accommodate the focal length dictated by the small aperture. Larger apertures would have resulted in too much aberration and a very indistinct image.
In 1757 John Dollond visited the workshop of George Bass, the lens grinder and polisher where he seems to have accidentally noticed the lenses of Mr Hall and discovered that flint glass held the solution. He conducted various trials with English crown glass, 'Venice' glass and English flint glass (white crystal), establishing that the last type had the most divergent properties. These experiments were written up by James Short through whom they were conveyed to the Royal Society.
Peter Dollond perusaded his father to seek to protect the discovery by means of a patent application. This was granted in 1758 and shortly afterwards the Royal Society awarded John Dollond its highest decoration - the Copley Medal.
Did you know? By the 1760s an achromatic telescope 3 feet in length could produce the same quality image as a non-achromatic telescope of 45 feet.
After John Dollond's death in 1761 the patent was frequently infringed. Contemporary trade literature shows that opticians such as Benjamin Martin or George Adams stocked both achromatic and 'ordinary' telescopes. Dollond's rivals felt justified in this as the story of Chester Moor Hall's earlier work became more widely known. In 1764 some 35 spectacle makers petitioned the crown to annul the patent and curiously the Worshipful Company of Spectacle Makers supported the petition against one of its own most eminent members. Emerging victorious from the case, however, Peter Dollond embarked upon a series of selective lawsuits against his principal opponents including James Champneys, Addison Smith and Francis Watkins. Through fierce protection of his patent rights Peter Dollond grew increasingly prosperous. The business relocated to St Paul's Churchyard in 1766. He developed the triple-lens achromatic telescope which resulted in an even finer image. The patent finally lapsed in 1772.
For a time in the eighteenth and nineteenth century the word 'Dollond' was almost a generic term for telescope rather like 'Hoover; is to vacuum cleaner. Genuine Dollond telescopes were considered to be amongst the best although there were several other London makers of the highest quality.
Peter Dollond (1731-1820) was the business brain behind the company which he founded in Vine Street, Spitalfields in 1750. The Dollonds were originally Huguenot refugees in the silk weaving trade so characteristic of that part of London. In 1752 he entered into a business partnership with his father John (1706-1761) and the shop was relocated to a position under the sign of the Golden Spectacles and Sea Quadrant in Exeter Exchange, off The Strand.
The Dollonds seem to have made both types of telescopes (reflecting and refracting), possessing the technology to produce significant numbers of lenses free of chromatic aberration for refracting telescopes by 1758, after which reflecting telescopes became less popular. Gregorian types of reflector are common enough, the rare examples being those that were not designed to be used with a stand. The dating of such items is often based mainly on the case (skin replacing leather in the mid 18th c.), where one exists.
In 1764 Leopold Mozart (father of Wolfgang) bought a Dollond 3-foot achromatic telescope with a double objective lens. Then, in 1766, Frederick the Great instructed his London ambassador to buy two telescopes from Dollond's 'because they magnify extraordinarily the object'. A Dollond telescope sailed with Captain Cook in 1769 on his voyage to observe the Transit of Venus.
In 1780 Dollond introduced the Army Telescope (or 'Improved achromatic telescope'). This is the standard type so often found by modern collectors with a mahogany body and brass draw-tubes. They were between 14 and 52 inches long with a lens aperture of between 1 and 2.75 inches. They cost from 2.5 to 12 guineas.
Thomas Jefferson visited P+J Dollond in 1786 and bought a telescope for 10 guineas. Admiral Lord Nelson would also make a special visit to purchase a Dollond.
Did you know?
Because Dollond & Co's records were destroyed by fire in the 1920s it is not usually possible to trace the lineage of individual telescopes, even when the serial number is present.
It might seem churlish to mention it, but 'Dollond' is actually spelt with an 'o' as the fifth letter. Many people get this wrong. If a telescope is marked 'Dolland' (with an ‘a’) then it is quite likely a counterfeit product. It may well be 19th c and a good example in its own right, but at the time it would have been considered inferior to the genuine branded article. Many manufacturers, some small and some not so small, sought to cash in on the Dollond name (with a letter 'o') - and marked their wares accordingly.
The advantage of reflectors was that they could be considerably shorter in length than the terrestrial refractor. Even after Dollond had solved the problem of chromatic aberration the company faced a constant struggle to obtain sufficient amounts of quality glass, resulting in the manufacture of telescopes with smaller objective lenses than might have been desired. In some surviving examples the eyepiece end has a wider diameter!
Peter Dollond often corresponded with Dr William Kitchiner. As well as a gourmet cook, Kitchiner was a lecturer on optics and author of The Economy of the Eyes (1824) and Spectacles, Opera Glasses and Theatres (1826). His memorial stone, recording his work on telescopes, may be viewed inside St Pancras Church in London. Kitchiner described Peter Dollond as 'The Father of Practical Optics'. In tests he conducted on ten telescopes the one made by Dollond allowed him to observe details which its rivals 'were not perfect enough to define at all'. Kitchiner collected over fifty telescopes in all and was not afraid to pass judgment. He once stated: 'Beyond a certain size, telescopes are only just as useful as the enormous spectacles which are suspended over the doors of opticians'.
Dollond & Co merged with Aitchison & Co in 1927 to form Dollond & Aitchison, the well-known high street chain of opticians, now fully part of Boots Opticians. They no longer manufacture but are exclusively a retail operation. The Company does not possess historic records of its former products and enquiries made directly to the company will normally be redirected to us here at the BOA Museum.
For more information on the company itself see the book by:
Hugh Barty-King, Eyes Right: The Story of Dollond & Aitchison, 1750-1985 (London: Quiller Press, 1986). A copy can be studied in the College library.
Kitchiner meanwhile provides evidence of the need for patrons at this period in order to further technological designs. A letter in our collection sent from Kitchiner to Sir Joseph Bankes, President of the Royal Society, in March 1819 states in quaint third person language that:
As soon as experi -ment, convinces him, that he has done all he can, Dr. K. will put down the curves of the lenses employed - and every particular relative to the construction of the eyetube - and have the honor of bringing the description, & the first instrument that is finished, to Sir Joseph Banks.
By May 1819 the psychophantic Kitchiner can report:
Dr K. has great satisfaction in informing Sir Joseph Banks - that his endeavors to improve the means of producing magnifying power at length completely successful - and the same tube now magnifies all the degrees - between 30 and 100 - in a day glass - and from 150 to 500 in a large astronomical telescope. Dr K has to thank Sir Joseph for the encouragement he gave to his contrivance in its incipient state - without that, he should never have had perseverance to produce the perfect instrument.
Our museum collection does not just concern English telescopes. Here are two items that represent the French side of the story in the nineteenth century. The first is an image from our Print Room, showing how telescopes could be bought from a street pedlar as well as from an optician's shop. There was an advantage to being able to try out the telescope outside!
The second item is a small ivory and brass telescope on display in the Giles Room. It extends into a short but nevertheless full-length telescope but compacted into an item small enough to carry in a coat pocket.
If your interest is in the smallest type of telescope, you may also enjoy our webpage about spyglasses.
Otherwise, take a peep into 400 Years of the Telescope (Part 2)