Tag Archives: catoptrics

The French Visitor

In 1785, the French engineer Joseph Cachin drew this delightful illustration of Bidston Lighthouse.

Drawings of Bidston Lighthouse and reflector, by Joseph Cachin, 1785.

Drawings of Bidston Lighthouse and reflector, by Joseph Cachin, 1785.

Cachin’s careful drawings include a scale, and one can take measurements from them. The scale is in toises and pieds du roi. These units were used in France before the Revolution. There were twelve pouces in one pied du roi, and six pieds du roi in one toise. Pouce, pied du roi and toise correspond respectively to the Imperial units inch, foot and fathom, but were slightly longer. Thus one toise was 1.066 fathoms, 6.394 feet, or 1.949 metres.

If you measure the diameter of the reflector in the drawing using Cachin’s scale, and convert back to Imperial units, you should get a value very close to 12 feet. This is the size that William Hutchinson reported in 1777 in his Treatise on Practical Seamanship, but less than the thirteen-and-a-half feet reported by Robert Stevenson when he visited Bidston Lighthouse in 1801. Perhaps Stevenson was more casual while he was still learning the trade of lighthouse-engineering, for he seems also to have exaggerated the height of Leasowe Lighthouse.

Bidston’s reflector was (probably) the largest parabolic reflector ever to be installed in an operational lighthouse. It was also one of the first – Hutchinson installed parabolic reflectors in all the Wirral lighthouses about the same time. Hutchinson’s breakthrough in lighthouse optics was driven by the needs of Bidston Lighthouse, which was built further from the body of water it lit than any other lighthouse in the world (a record it holds to this day).

As far as I know, Cachin’s is the only drawing to show a cross section of the lamp room, with the stairs that the keeper would climb to replenish the oil reservoir, and the elaborate chimney through which the smoke and vapours of the lamp would escape.

Joseph Cachin (1757-1825) was a French engineer, best known for his work at Cherbourg Harbour. In 1785, while at the École Nationale des Ponts et Chaussées, he travelled to Britain and the United States. In England, he visited the ports of Sheerness, Liverpool, Bristol, Portsmouth, the quarries of Portland, and the Hospital at Greenwich. He examined lighthouses and lime-kilns, the cranes on the Bridgewater Canal, and studied the materials used in their construction. (Readers of French can check my translation against this article.)

My thanks to Thomas Tag of the United States Lighthouse Society for bringing the drawing to my attention.


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.