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Augustin Fresnel is the most celebrated example of a French scientist applying his discipline in the service of lighthouses. Fresnel was a student at both the École Polytechnique and École Nationale des Ponts et Chaussées, where he attained distinctions for his work. He was appointed as Engineer successively to the departments of Vendée on the Atlantic coast, Drôme in the south-east and Ille-et-Vilaine in Brittany. Soon after these appointments Fresnel set up light tests of Bordier-Marcet’s fanal sidéral apparatus, destined for Les Baleines lighthouse, at the Arc d’Etoile in Paris, so that members of the Lighthouse Commission could view its effectiveness.

Politics wrecked Fresnel’s career for a short time because he had supported the Bourbon monarchy and he lost his appointment when Napoleon returned to power. As the political scene changed, on the second restoration of the monarchy in 1815, he obtained a post as engineer in Paris, where his life from that time was spent researching optics.

Fresnel attained fame for introducing the wave theory of light, which countered the corpuscular theory in vogue since the days of Newton 100 years earlier. Debate over the merits of the two theories assumed a high profile when David Brewster opposed the wave theory, which, after Thomas Young’s work paralleling Fresnel’s, had received general acceptance in Britain. Brewster’s anti-establishment stance put him out of favour with the British scientific community and led to much of his work, including his advocation of the use of lenses in lighthouses, being ignored. (This is taken up in more detail in the next chapter.) Meanwhile in 1819 Fresnel was appointed Secretary of the Lighthouse Commission until his death in 1827. It was no accident that his research was steering in the direction of lighthouse illumination. The Astronomer Royal, François Arago, supported Fresnel’s optical theories, working with him to confirm Fresnel’s wave theory of light, observing what is now known as the ‘spot of Arago’. This was the foundation of an intimate friendship between Arago and Fresnel, which led Arago, as a member of the appointments committee of the Lighthouse Commission, to suggest both the creation of the post of secretary and his friend as the incumbent.

Fresnel was a multi-talented physicist and engineer who worked on wide-ranging projects. They were as diverse as improving methods to manufacture sulphuric acid, improving roads and bridges, and developing optical devices. In 1819, at the request of Arago, Fresnel was invited to conduct ‘the necessary experiments and researches’ into methods of lighthouse illumination. Fresnel’s paper presented to the Lighthouse Commission described what he called the annular and later renamed dioptric lens, which was produced for the first time in 1821, by the Parisian optician Francois Soleil. The following year Fresnel improved on his first design by describing a lens made of eight bull’s-eye flash panels in his paper ‘Memoire sur un Nouveau Systeme d’Eclairage des Phares’. James Chance gave due recognition to the Commission in his 1867 paper ‘Optical apparatus of lighthouses’ presented to the Institution of Civil Engineers:

It is, indeed, creditable to the Administration in France that her highest men of science should be thus enlisted in the investigation of a national question requiring scientific treatment, and the result in this instance proved the wisdom of their selection.

In 1822, the Commission confirmed the adoption of Fresnel’s system and presented a programme for the systematic lighting of the coasts and harbours of France, ‘Rapport contenant l’exposition du système adopté par la commission des phares pour éclairer les côtes de France’ (‘Report containing the description of a system adopted by the lighthouse commission for lighting the coasts of France’). James Chance showed his preference for the new system over the outmoded catoptric system in his 1867 paper:

This scheme was gradually carried into effect, and so strictly has it been adhered to, that out of forty-nine sea lights which were proposed, only ten have been modified in their character, and the employment of metallic reflectors in sea lights has been reduced to the single instance of a secondary lighthouse at Pontaillac, at the mouth of the Gironde.

The word ‘dioptric’ is derived from two Greek words meaning ‘through’ and ‘to see’. Fresnel’s first dioptric lens array was made by placing a number of lens panels round a central light source. The common focus then produced a concentrated beam of light projected through each lens panel. The lens panels were made up of a central lens (a bull’s eye in common parlance), surrounded by a series of concentric segmental prismatic lenses each ground to the profile of the central lens with the same focal length. Each element was then cemented together into a supporting metal frame. The complete assembly with its oil wick burner was mounted on a clockwork-driven carriage if the light was to be revolving or set in a metal frame if fixed. In the revolving variety, as the whole array revolved slowly each beam appeared as a flash as it passed the observer’s eye. Fresnel’s dioptric system included a detailed specification of six ‘orders’ or sizes of lens. The largest was the first order, with a focal distance of 920mm (36.2in), used where the light needed maximum power to reach the horizon and if set high above the sea was visible to ships up to 30 miles (48km) distant. The orders then reduced in size – a second order having a focal distance of 700mm (27.5in), a third order 500mm (19.6in), a fourth order 250mm (9.8in), a fifth order 187.5mm (7.3in) and a sixth order 150mm (5.9in).

Fig 4

Fresnel’s polyzonal dioptric lens with its bull’s eye and concentric rings. The lens captures the light from a lamp then focuses it into a cylinder or beam of light projected to the horizon.

Another important variant in Fresnel’s design was the use of a three or four-wick oil burner in place of the standard Argand burner. With the burner now being placed behind the lens it was important to have a single source of light, while a typical catoptric apparatus had anything up to 30 reflectors, each with its own Argand burner and each producing a relatively small flame. The Fresnel burner therefore needed to be much bigger, and consumed about 1,000 gallons (4,546 litres) of oil per year, whereas an equivalent catoptric apparatus such as the 30-reflector light at Beachy Head in England consumed about 785 gallons (3,568 litres). Fresnel’s burner used a clockwork device to supply oil from a reservoir to the wick, and relied on an overflow of oil both to keep the temperature of the oil down and to prevent carbonization of the wick. This factor was extremely important in the tests that were carried out by Michael Faraday, Sir George Airy and James Chance in 1860, when they were trying to work out why the French dioptric lights performed so much better than their English counterparts.