Meeting of the Historical Section: held at the BAA Winchester weekend, Sparsholt College, Hants. on 2013 April 6.
The first speaker was Bob Marriott, Director of the Instruments and Imaging Section. His talk was entitled 'The silver-on-glass revolution' the arrival in the 19th century of glass telescope mirrors coated with silver. This, Bob said, was a revolution for amateur astronomers because, almost suddenly, they could make large and small telescopes much more conveniently and at a lower cost, though the silvering process itself evolved over several decades.
Since the invention of the reflecting telescope in the 17th century, mirrors had been made of speculum metal--an alloy of copper and tin that could be polished to produce a reflective surface. Speculum metal is much heavier than glass and tarnishes quickly, and mirrors required frequent repolishing. The process of production of silver involving silver salts, aldehydes, and sugars was discovered in the early 1830s by the German chemist Justus von Liebig, whose work involved the application of chemistry to agriculture and industry.
The silvering process was developed over many years and numerous patents were registered by many researchers, but it was applied primarily for decoration and ornamentation and it was not until the 1850s that a sufficiently fine and stable silver deposit could be produced that was good enough for astronomical purposes. The first telescope with a silver-on-glass mirror was built in France by J. B. L. Foucault in 1857. Two years later, Henry Cooper Key and George With, both of Hereford, made the first silver-on-glass mirrors in England, and in the early 1860s George Calver began commercial production of silvered mirrors. Over many years, With produced about two hundred mirrors, while Calver established a business and produced countless numbers of mirrors and complete telescopes--a business that he maintained for sixty-five years until his death at the age of 93 in 1927.
The year 1859 was notable for many other significant developments in science, literature, philosophy, politics, and social perception and reform, with the publication of fundamental and epochal works by Charles Darwin, John Stuart Mill, Karl Marx, Samuel Smiles, Florence Nightingale, Isabella Beeton, and many others--including T. W. Webb's Celestial Objects for Common Telescopes. Notably for astronomy, Kirchhoff and Bunsen produced the first map of the normal solar spectrum and instigated the new science of astrophysics. In addition, in that same year Henry Bessemer began the production of steel using his revolutionary new process of mass production, initiating the second Industrial Revolution; later, he built an observatory housing a 50-inch reflector with a mirror by Calver.
The deposition of silver on glass remained the standard technology for telescope mirrors for seventy years, until the advent of aluminium coatings around 1930.
In addition to Bob's talk, Lorraine Crook brought mirror-making materials and throughout the weekend many participants tried their hand at grinding two pieces of glass together.
Deputy Section Director Lee Macdonald then spoke about 'Isaac Roberts, E. E. Barnard and the Mysterious Nebulae'. Born in 1829, Isaac Roberts was an amateur astronomer who made his fortune in the Liverpool building trade. In 1885 he began photographing star clusters and nebulae using a 20-inch reflector, first from near Liverpool, then after 1890 from Crowborough in Sussex, where Roberts moved in search of clearer skies.
Roberts was the first deep-sky photographer to go really 'deep'--that is, to use very long exposures to reveal faint nebulosity unsuspected by even the best visual observers. His 1888 photographs of M31 in Andromeda were sensational: they showed for the first time that this object was, in fact, a 'spiral nebula' highly inclined to our line of sight, and at first considerably strengthened the theory that spiral nebulae were not 'island universes' but relatively nearby objects, perhaps solar systems in the process of formation. However, if they were nearby, the spirals should show detectable internal motion within a short period, and Roberts carefully measured his own photographs of M31 and several other objects taken several years apart. He could find no such motions and concluded that the spirals were very distant, though he did not come round in favour of the island universe hypothesis.
Roberts soon had a rival on the other side of the Atlantic. Edward Emerson Barnard came from a very poor background and had no formal scientific qualifications, but he established a reputation as a first-class observer and gained employment at the newly-opened Lick Observatory in California in 1888. He quickly began taking long-exposure photographs of nebulae.
In 1895, Roberts questioned the reality of nebulosity that Barnard had photographed around the star 15 Monocerotis, which Roberts could not discern on his own three-hour exposure of the same region. Roberts thought that the alleged nebulosity was background stars that were not resolved in Barnard's smaller instrument. Three years later, Roberts similarly dismissed the apparent presence of faint outer nebulosity near the Pleiades on a photograph by Barnard. Finally, in 1903, Roberts claimed from his own photographic survey that only four large 'nebulous regions' originally catalogued by William Herschel actually contained any nebulosity. Once more he dismissed as spurious nebulosity photographed in several other regions by Barnard and other astronomers.
In each case, Roberts never changed his position on the existence of the nebulae, despite growing evidence that they were real. One historian has used one of these clashes to imply that Roberts was amateurish in his methods, but evidence from primary sources suggests that he was, in fact, a careful and methodical worker. Moreover, some correspondence between Roberts and the Royal Astronomical Society shows that Roberts' intransigence in the face of evidence that he was in the wrong was due less to any lack of 'professionalism' on his part than to an irascible temperament and aversion to criticism--an aversion that was probably shared by Barnard.
Yet whether we think of them as 'amateurs' or 'professionals', Roberts and then Barnard convinced leading astronomers of the day that nebulosity had to be taken seriously. It was not long after their clashes in the 1890s that astronomers in the western United States were using large telescopes to photograph vast numbers of 'nebulae' and take their spectra, a process of discovery that would eventually lead to Edwin Hubble's determination of the distance to M31 and establishing that spiral nebulae were indeed 'island universes'. Thanks to the work of Roberts and Barnard, 'nebulae' were no longer mere fuzzy curiosities in the sky, but important objects that could be used to test theories of cosmology.
Following afternoon tea, Paul Haley gave the final talk of the day, on 'Sir David Gill (1843-1914), Surveyor to the Stars'. Paul was in charge of 'Gill-100', a project funded by the RAS and other organisations to transcribe Gill's vast correspondence with the leading astronomers of his time. Participants were able to view a fine exhibition of Gill letters and photographs that Paul had brought to the weekend.
David Gill was born in Aberdeen, the son of a clockmaker in the city centre. From 1858 to 1860 he was a student at Marischal College, now part of the University of Aberdeen. The great physicist James Clerk Maxwell, Professor of Natural Philosophy at Aberdeen, described Gill as 'one of my ablest students at Marischal College'. Gill's father wanted him to go into the family clock business, so he served an apprenticeship there, then worked in Coventry, Clerkenwell and even Besancon in France before returning to Aberdeen. He later came to value his workshop experience in making precision instruments, but his real passion was astronomy. He was taught observational astronomy by another Aberdeen professor, David Thomson, in the university observatory.
By the late 1860s Gill had a small observatory of his own in his father's back garden, equipped with a 12-inch Newtonian that had one of the new silver-on-glass mirrors figured by Henry Cooper Key (see the report of Bob Marriott's talk above). In 1868 he took a very fine image of the Moon that greatly impressed Lord Lindsay, an Aberdeenshire landowner and amateur astronomer. Lindsay offered Gill the job of astronomer in the very well-equipped observatory on his Dun Echt estate. Gill gladly accepted, for this allowed him to leave the family business and start a career as a professional astronomer. In 1874 he accompanied Lindsay to Mauritius to observe that year's transit of Venus. Gill's observations were highly successful and he also did a geodetic survey of Egypt on the way home.
Unfortunately, Gill's success as an astronomer aroused Lindsay's jealousy and Lindsay's mother also resented the prominence of this modest middle-class man on the family estate. Gill, who by now had a young family to support, was made redundant from Dun Echt, but fortunately, he was not unemployed for long. Aware of the limited accuracy with which observations of transits of Venus could be used to measure the Earth-Sun distance, Gill made an expedition to Ascension Island to measure this important constant using the parallax of Mars. His success led to his appointment at the Royal Observatory at the Cape of Good Hope, where he spent the rest of his working life.
From South Africa Gill took a photograph of the great comet of 1882 that showed not only the comet itself but also vast numbers of stars. This image convinced many astronomers of the potential of photography in precision mapping of the stars. In 1887, many of the world's leading astronomers gathered in Paris to launch the 'Carte du Ciel', an international project to assemble the most detailed star catalogue ever produced, using photography with a set of standardised telescopes in various observatories around the world.
Mike Frost concluded the meeting by thanking all the speakers and the organisers of the Winchester Weekend, both in the BAA and at Sparsholt College.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||BAA Update; British Astronomical Association|
|Publication:||Journal of the British Astronomical Association|
|Date:||Aug 1, 2013|
|Previous Article:||Summer supernova remnants.|
|Next Article:||Meeting of the Comet Section: held at the Humfrey Rooms, Northampton on 2013 May 18.|