Читать книгу Lighthouses - Peter Williams - Страница 17

By 1820, reflector or catoptric apparatus was the state of the art in lighthouse illumination both in Britain and the rest of the world. The idea of using glass prisms to condense and magnify the light rather than parabolic mirrors to reflect it required completely new thinking, which was alien to the traditions favoured by Trinity House and the other British lighthouse authorities. This revolution evolved from an unrelated application in the 1740s – the use of lenses as burning instruments – to becoming a practical reality more than 80 years later. Almost all the pioneering work was done by French scientists and noble engineering dabblers, though in Scotland David Brewster (see above) was working on a very similar idea. The application of scientific knowledge to lighthouse illumination occurred in fits and starts and only became a recognized discipline after Augustin Fresnel published his paper on the dioptric lens in 1819. This story provides an insight into the contrasting approaches to science and invention practiced in France and Britain, and leads to the situation in the 1830s when Scotland, under the direction of the Northern Lighthouse Board, had begun to catch up with France, while in England Trinity House was sceptical of the new illumination technologies and chose to stick with the catoptric system.

In France in 1748, Georges-Louis Leclerc, Count de Buffon, carried out the first experiments with lenses when he suggested fabricating lens segments, or echelons, to reduce the thickness of the glass necessary to produce the desired concentration of light. A single piece of uncut glass absorbed too much light for it to be useful. Buffon was interested in the heat his lens could generate, used in experiments where chemicals needed to be heated to high temperatures to reduce them to their elements. If a 24-in (61-cm) diameter x 3-in (7.6-cm) thick lens was needed this could be made better if each element was only 1in (2.5cm) thick to allow better transmission of light. The lens blank was laboriously ground in three steps. The central element was an 8-in (20.3-cm) diameter disc, the intermediate element an 8–16-in (20.3–40.6-cm) diameter circle, and the outer element a 16–24-in (40.6–61-cm) diameter circle. The completed lens was 3in (7.6cm) deep but at no point did the rays pass through glass thicker than 1¹/₂in (3.8cm). Wrote Buffon:

If a piece of glass four feet in diameter by two and one-half inches thick be first moulded and cut into steps (echelons) for a focal distance of eight feet, I have estimated that, leaving it even one and one-half inches think at the centre of the lens and the same at the interior edges of the steps, the heat from it (used as a burning glass) will be to that of the Palais Royal lens, as 28 is to 6, without taking into account the effect due to the difference of thickness, which is very considerable, and which I can not estimate in advance.

Thomas A. Tag, Who Invented the Fresnel Lens?

Buffon’s lens was only built for the first time more than 30 years later by the Abbe Rochon, but on a much smaller scale than Buffon’s design proposed. A modified version of Buffon’s lens was suggested by the mathematician and philosopher the Marquis de Condorcet in 1788, made from a series of circular lenses cut in separate pieces, but no lens of this design was ever built.

Coincidentally, the first recorded use of a glass lens in a lighthouse also took place in 1788 but this time in England, when Thomas Rogers installed a bottle-glass plano-convex lens in front of the oil lamp and a spherical reflector behind it at Portland, Dorset. Neither this nor his other attempts at North Foreland in Kent, Liverpool and Dublin were successful. The 20-in (50.8-cm) diameter, 5-in (12.7-cm) thick lens did not perform as planned but in fact decreased the light. Lieutenant Thomas Drummond of the Royal Engineers, who in 1834 was working with the Trinity House tests on dioptric apparatus, referred to the lens as ‘consisting of one solid piece of glass, very thick and very bad’. The next attempt to use lenses was by George Robinson in the Flamborough Head and South Stack lighthouses in 1806, but they only lasted a few years before being replaced with reflectors. In 1812, the American Winslow Lewis, after seeing Robinson’s light at South Stack, sold the patent rights to the US government to what he called his ‘reflecting and magnifying lantern’. He convinced the authorities to install this in more than 50 lighthouses during the next 40 years. Hoodwinked by the apparatus’s use of half the oil of conventional designs, the US authorities finally sent a delegation to France, which observed the superiority of Fresnel’s lenses and recommended a wholesale replacement of the poorly performing Lewis system.

In 1812, David Brewster enters the story for the first time. He wasn’t to know that this was the start of a 50-year love-hate relationship with the British lighthouse authorities and the Stevenson family in particular. Brewster was born in 1781 into a humble family and at the age of 12 was enrolled into the University of Edinburgh to study for a ministry in the Church of Scotland. Graduating with an honorary Master of Arts degree at the age of 19, his early attempts at preaching revealed his failings as a public speaker, so he turned his attention to science and particularly the science of optics. By 1808, his work had earned him recognition by the Royal Society of Scotland, which elected him a fellow, and in the same year he embarked on a career in publishing, which proved to be his most lasting pursuit, being appointed editor of the Edinburgh Encyclopaedia in 1807.

Brewster’s 1812 paper on burning instruments credited Buffon and expanded his designs into a detailed system that used both reflection and refraction to capture sunlight and produce a single and far more powerful beam. Though Brewster was unaware of Condorcet’s idea of modifying Buffon’s lens into segments, his burning instrument also entailed separating Buffon’s glass echelons into segments, which could be more easily cast and ground. Like its predecessors, the lens segments were to be arranged next to each other forming concentric circles with a bull’s-eye lens in the middle. Brewster combined this with an array of mirrors, which reflected the light that could not be captured by the lens into the same beam of light produced by the lens. Brewster at this stage did not envisage his lens as an alternative to reflectors for lighthouses, but the combination of the arrangement of lenses and mirrors to produce a cylinder of light was later adopted by all lighthouse optical engineers. It became a source of exceedingly acute irritation to Brewster that the British lighthouse fraternity did not take up his idea right away, and he himself did not make the connection with lighthouse illumination. Nobody else in Britain did either. If anyone at Trinity House or the Northern Lighthouse Board did read his paper they were still stuck in the reflector mindset, which is odd considering Trinity House’s previous experiments with the Wilkins/Rogers (see above) lenses.

Brewster sent a copy of his paper to the French Institute, where optical science was taken very seriously. The Institute had close relations with the French lighthouse authority, though it has never been established whether Fresnel read Brewster’s paper, and he makes no mention of it in any of his treatises on the subject. In 1813, Brewster published his first major work on optics, a Treatise on New Philosophical Instruments for various purposes in the arts and sciences with experiments on light and colour. The book concentrated on eyeglasses, opera glasses, night glasses, telescopes, microscopes and micrometers, the devices in which British optical scientists were most interested. He included a drawing at the back of the book showing methods for measuring distances using optical instruments, with a tower located on a cliff top looking over a landscape, but no sea is depicted. Had he been thinking about lenses for lighthouses this is surely where he would have written about it.