Light and Lens
The History of Lighthouses
Course 8 · Chapter 5 · Light and Lens
A tower is only the support for a light, and the light itself went through a long series of improvements: from open fires to candles and oil lamps, from bare flames to polished reflectors, and from reflectors to the great glass lenses that made a single lamp visible for twenty miles. This chapter follows the history of the light, the lens of Augustin Fresnel and its adoption in Britain, the system of flash patterns that lets a sailor identify a light, and the fog signals that did the same job by sound. Names, dates and measurements are from general knowledge and flagged "check".
From Fire to Lamp
| Stage (check each) | Detail |
|---|---|
| Open fires | Wood and coal burned in an iron basket on a tower or hill. Bright but smoky, wasteful and hard to keep burning in wind and rain (Chapter 2). |
| Candles | Smeaton's Eddystone of 1759 was lit by a ring of tallow candles, which needed constant snuffing (check). |
| Oil lamps and reflectors | Lamps burning whale or vegetable oil with a polished reflector behind each flame were used by the late 18th century. The Argand lamp, with a hollow circular wick that drew in air to give a steadier and brighter flame, was introduced in the 1780s (check). Thomas Smith used these at Kinnaird Head (Chapter 4). |
| Fuels | Sperm-whale oil gave way to colza (rapeseed) oil in the 19th century, and then to paraffin from the 1870s (check). Later came the incandescent oil-vapour burner, which gave a much brighter light with the same fuel. |
| Gas | Acetylene gas, with a device that turned the light off in daylight, was developed by the Swedish engineer Gustaf Dalén and won him a Nobel Prize in 1912 (check). It made unstaffed lights possible in remote places. |
| Electricity | The South Foreland light in Kent was lit electrically in 1858, with the advice of Michael Faraday, and the system was tried at other stations in the following decades (check). |
Fresnel and the Lens
The problem
A reflector wastes much of the light and loses more as the metal tarnishes. Engineers wanted a way to gather all the light from a lamp and send it out in a narrow beam.
The solution
Augustin-Jean Fresnel (1788 to 1827), a French engineer and physicist, designed a lens made of concentric rings of glass, each ring cut to bend the light the same way. It gave the effect of a huge lens without the thickness and weight (check). Prisms above and below caught more of the light.
The first use
The first Fresnel lens in service was installed at the Cordouan lighthouse at the mouth of the Gironde river in France in 1823 (check). Lenses were made in sizes called orders, from the largest (first order) to the smallest (sixth order).
The Lens in Britain
- The Stevensons. Alan Stevenson visited France to study the new lenses and used a first-order lens at Skerryvore (Chapter 4, check). Scottish and English lights adopted the system through the 1830s and 1840s.
- The makers. British lenses were made by the glass firm Chance Brothers in the English Midlands, which became a leading supplier around the world (check).
- Improvements. Thomas Stevenson developed "holophotal" optics, which gathered light from all directions, and worked on improving the efficiency of the lens (check).
- The lens floating on mercury. A rotating lens could weigh several tonnes. Many were floated on a bath of mercury so that a keeper could turn them with a finger, a practice later abandoned because of the health risk of mercury (check).
How a Light Tells Who It Is
| Characteristic (check each) | Meaning |
|---|---|
| Fixed | A steady light with no change. |
| Flashing | Flashes of light separated by longer periods of darkness. |
| Occulting | A steady light that is regularly cut off for a short time. |
| Group flashing | Groups of flashes at regular intervals, such as two flashes every 15 seconds. |
| Colour | White, red or green, used by themselves or in sectors to show safe and dangerous areas. |
| Period | The time from the start of one pattern to the start of the next. |
Charts and "lists of lights" give the characteristic of every light, so that a navigator can recognise a light by counting the flashes with a watch. The range at which a light can be seen depends on the height of the light above the sea. As a rough rule, the distance to the horizon in nautical miles is about 1.17 times the square root of the height in feet (check). A light at 100 feet can be seen from about 11 or 12 nautical miles for an observer at sea level.
Sound in the Fog
- Bells. Fog bells, rung by hand or by clockwork, were used from the early 19th century. The Bell Rock had a bell on its tower (check).
- Guns and rockets. Some stations fired a gun at set intervals, which was loud but costly.
- Sirens and horns. Steam, compressed air and, later, electric horns replaced bells from the mid-19th century (check). The characteristic sound of a diaphone is remembered by many.
- Limits. Sound travels unevenly in fog, and the range is short, so the sound signal gave a warning but not a precise bearing.
Exercises
Test the claim "Fresnel invented the lighthouse lens" with the six questions and give a more careful sentence.
Using the rough rule in this chapter, estimate how far away the horizon is for a light 80 feet above the sea, and say why the real range of the light may differ.
Describe in words the light shown by a station whose chart entry reads "Fl(2) W 15s", and explain why a navigator needs a watch to use it.
Quick Reference
- Light: fire, candle, oil lamp with reflector, Argand lamp, paraffin, acetylene, electricity
- Lens: Fresnel, first used at Cordouan in 1823; orders from first to sixth (check)
- Pattern: fixed, flashing, occulting, group flashing, colour and sectors
- Sound: bells, guns, sirens and horns