Читать книгу Lighthouses - Peter Williams - Страница 14

Optical glass starts as a pot of glass, which is left to cool very slowly to obtain the required homogeneity. Early work to produce good quality optical glass was carried out by a Swiss woodcarver, Pierre Louis Guinand, in 1768. By 1775, he was able to demonstrate lenses for telescopes, using a composition of flint glass (combining sand, potash and lead oxide), which was heated and stirred in ways that reduced the striations (lines) in the glass, thus improving its optical qualities. Shortly before Guinand’s death, he negotiated unsuccessfully with the French government for assistance and his work went nowhere, though importantly he did leave detailed records that were later to prove crucial to Chance Brothers. Beginning in the late 18th century, Britain had lost its lead in the manufacture of telescopes to Germany. Here, the work of Joseph von Fraunhofer in centralizing the manufacture of glass and instruments along scientific lines between 1805 and 1825 was not matched by the English opticians, who did not have a reliable supply of optical quality glass. The efforts of English opticians W.V. Harcourt, G.G. Stokes and others to rectify this proved fruitless. Michael Faraday related in 1829 how the English optician John Dollond ‘had not been able to obtain a disc of flint glass four and a half inches in diameter, fit for a telescope, within the last five years, or a suitable disc of five inches in diameter within the last ten years.’ The testimony of Sir George Airy, Astronomer Royal, is further evidence of the difficulties experienced at this time. In 1880, he wrote to James Chance, recalling that in about 1828, when he was at Cambridge, he ‘had to wait for years for a 4-inch object of glass.’ What was lacking in Britain was collaboration between the optician, the scientist and the glass technologist.

In 1824, the Royal Society appointed a Committee of its Fellows and members of the Board of Longitude to look into rectifying this problem. Michael Faraday, Sir John Hershel and others formed a sub-committee to experiment with various mixtures of glass and techniques of heating and cooling the glass. The London glassmaker Pellatt and Green was commissioned to build a furnace for the project, but Faraday found this to be too inconvenient, so a small furnace was set up at the Royal Institution. From 1827 for the next three years, Faraday spent much of his time trying to produce good quality optical glass, writing a paper for the Royal Society ‘On the manufacture of glass for optical purposes’ in 1829. It proved to be a frustrating time for Faraday and in 1831 he wrote to the Royal Society requesting that he ‘lay the glass aside for a while, that I may enjoy the pleasure of working out my own thoughts on other subjects’. The failure of the Royal Society and Faraday’s experiments indicates how they underestimated the difficulty of producing optical glass. Though Faraday was undoubtedly a talented experimental scientist, the task was beyond even his capabilities.

Meanwhile, across the English Channel, George Bontemps was one of the French glassmakers attracted by the French Academy of Science’s offer of a prize for the best optical glass produced by a French glassmaker. Bontemps was put in touch with the eldest son of Pierre Louis Guinand, Henri, who wanted to sell his father’s secrets. Bontemps – one of the most skilful glass technicians in France – purchased them for 3,000 francs (£5,290 in today’s money) in 1827. After initial attempts at his glassworks in Choisy le Roi near Paris failed, Bontemps, after parting ways with Henri Guinand, decided to persevere alone. By 1828, he was ready to present to the French Academy some lens discs that had been made using better stirring methods to obtain a high-quality flint glass. Bontemps was later to come to Chance Brothers to direct their optical glass works and this enabled the firm to extend their operations to the making of optical quality glass for lighthouse lenses in 1849.