Catoptrics

William Hutchinson, Liverpool Dockmaster, revolutionised lighthouse optics with the introduction of the parabolic reflector. He conducted experiments at the Bidston Signals Station during the 1760s, and subsequently installed reflectors in the Wirral Lighthouses. He writes, in his Treatise on Practical Seamanship:

“We have made, and in use here, at Liverpool, reflectors of one, two, and three feet focus ; and three, five and a half, seven and a half, and twelve feet diameter ; the three smallest being made of tin plates, soldered together ; and the largest of wood, covered with plates of looking glass”.

Hutchinson installed the largest one at Bidston Lighthouse, which was furthest from the sea. The large reflectors had some problems: they used a lot of oil, and smoke from the wick tended to obscure the reflector. Eventually, it was found that several smaller reflectors, arranged so that their beams were parallel, gave an equally bright light and used less oil.

Trinity House have an eighteenth century parabolic reflector in their collection. Its diameter is twenty inches.

18th century catoptric reflector, in the Trinity House collection.

18th century catoptric reflector, in the Trinity House collection. Photograph courtesy of the Corporation of Trinity House.

Little is known about the provenance of this reflector, because, sadly, Trinity House was bombed in 1940, and many of its records destroyed. The reflector will be on display at the National Maritime Museum in Greenwich from March 2014 until January 2016.

Trinity House is 500 years old this year. Henry VIII granted a Royal Charter to Trinity House in 1514.

Most of the lighthouses of Liverpool Bay were built and run by the port of Liverpool, not Trinity House. The Mersey Docks and Harbour Board took over from the Liverpool Dock Trustees in 1858. It was not until 1973 that Trinity House took over Hilbre Island Lighthouse and Point Lynas Lighthouse from the Mersey Docks and Harbour Board.

Catoptric lights are based on the principle of reflection. They were eventually superseded by dioptric lights, based on the principle of refraction, thanks to the work of Augustin-Jean Fresnel. When Bidston Lighthouse was re-built in 1873, it was equipped with a Fresnel lens manufactured by Chance Brothers.

Acknowledgments. My thanks to the Corporation of Trinity House and Neil Jones, Archivist, for permission to use the photograph of the catoptric reflector.

Share this:
Facebooktwittergoogle_pluspinterestlinkedinmail

2 thoughts on “Catoptrics

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.