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‘Fourth order lighthouse at Penfield Reef, Long Island Sound’. From _Illustrated Science for Boys and Girls_, 1881. Author unknown. Project Gutenberg ebook.

Since I just came back from a vacation in Delaware and Maryland, this 1881 illustration of a lighthouse seemed appropriate. Lighthouses of this period used Fresnel lenses, invented in 1822. These are the distinct ‘rippled’ lenses in 19th c. lighthouses that focused 85% of the lamp’s light (compared to the pre-Fresnel 20%). Though they weren’t introduced in the US until the 1850s, by 1860 all US lighthouses had them.

Fresnel lenses were ranked in ‘orders’ – with ‘first order’ being the largest, followed by ‘second order’, and so on down to ‘sixth order’. The caption of the illustration refers to it being a ‘fourth order’ lens. In modern lighthouses, Fresnel lenses are often replaced with modern alternatives that require less maintenance.

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