A Light to Lighten our Darkness: Lighthouse Optics and the Later Development of Fresnel's Revolutionary Refracting Lens 1780–1900

Abstract

This paper on lighthouse optics describes the background to Augustin Fresnel's concept of refracting, or dioptric, built-up glass lenses, which superseded the catoptric or reflecting system using silvered copper mirrors. Fresnel's brilliant invention revolutionized navigation and coastal lighting. The paper focuses on the period after Fresnel's death in 1827, which occurred before he had time to perfect his optical instruments. It describes the achievements of Fresnel's successors in France and Britain who were finally able to realise the full potential of the dioptric lens for both fixed and revolving lights, ranging from small harbour lanterns to the great sea lights. Léonor Fresnel, Augustin's brother, and Alan Stevenson of the Scottish lighthouse family eventually succeeded in producing the first all-glass optic. This incorporated large-scale internally reflecting or catadioptric prisms, which extended the lens action of the original dioptric lens panels and harnessed more of the light rays. Thomas Stevenson built on this work to design his “Holophote” or “Light of maximum intensity”, a new type of catadioptric lens. Thomas went on to create increasingly sophisticated instruments, such as the “Azimuthal Condensing Light”, designing new forms of prism to direct beams of light with increasing accuracy, maximizing strength and brightness and minimizing fuel consumption. Working alongside the engineers was a group of specialist manufacturers who developed the fabrication techniques to make these increasingly large and complex instruments. Particularly notable was the firm founded by François Soleil who had worked with Augustin Fresnel on the earliest lenses, while Soleil's son-in-law, François Jeune, collaborated with Léonor Fresnel and Alan Stevenson. Chance Brothers of Birmingham was the other major optic manufacturer, thanks to the achievements of James Timmins Chance, who worked closely with Thomas Stevenson to produce such useful devices as the dioptric mirror, and with Michael Faraday to transform the accurate setting and adjusting of the component parts of each optic. Chance's successor in the firm, John Hopkinson, invented “Group flashing”, which introduced further categories of distinctions by which each lighthouse could be identified. Towards the end of the nineteenth century, advances in physiology led to a greater understanding of the effect of a flashing light on the human retina. This led to a new generation of optics with higher rotational speeds made possible by the introduction of the mercury float. Scientists such as Bloch and Charpentier worked in parallel with the lighthouse engineers, notably Bourdelles, Blondel and Rey, using these new findings to design smaller but more powerful and efficient lenses, particularly suitable for electricity.