One of the marquee exhibits at the Maine Maritime Museum in Bath is an original Fresnel lens from the Two Lights lighthouse in Cape Elizabeth.

According to Pressherald.com, the lens weighs 1,800 pounds and is valued at $2.5 million. The Coast Guard retains ownership of lighthouse lenses, which have mostly been taken out of use, and it loans them out to museums under strict conditions. Their lending guidelines describe the lenses as extremely fragile artifacts which must be protected from direct sunlight and UV radiation. They must be protected from vibration and humidity and kept within a narrow temperature range. This begs the question: if light, temperature change, vibration and humidity are harmful to the lens, how in the world did it survive at the top of a Maine lighthouse for 120 years?

The book “A Short Bright Flash” by Theresa Levitt describes the dangerous world of early navigation. Lighthouses were few and far between in the 18th century, and their dim light, reflected by mirrors, could only be seen from a few miles away. Rather than warning ships about navigational hazards, which could often be far offshore, the early lighthouses usually marked the entrance to harbors. The heyday of lighthouses only came after the invention of the Fresnel lens, which could gather light from a lamp and project it up to 20 miles in a tight beam.

Augustin Fresnel, born in France in 1788, started out in road construction, according to Britannica.com. Soon he became involved in scientific experimentation with light. Although he is known today for his lenses, his real work involved trying to discover the true nature of light. Isaac Newton’s theory was that light was made of particles, but newer theories suggested that light was made of waves. Fresnel agreed with the wave theory, but there were obvious flaws with it that could not be explained. Eventually, he realized that light must not be made of longitudinal waves, as previously thought, but must consist of transverse waves. This new discovery earned him an award from the French Academy of Sciences in 1819.

Hired to improve France’s lighthouses, Fresnel invented his famous lens. A conventional lens could have been used to focus a lighthouse beam, but such a lens would have been extremely thick and heavy. Fresnel’s lens, which consisted of small lenses surrounded by many curved prisms, achieved the same effect with a far thinner and lighter profile. Connecting several Fresnel lenses and rotating them allowed a lighthouse to throw multiple beams from the same light source. This invention led to an explosion in lighthouse building around the world, including here in Maine. Lighthouses could now be used to warn ships away from ledges and sandbars miles from the coast, and countless ships and sailors were saved. Today Fresnel lenses are used in many applications, including automobile lights, and can even be made into thin sheets that magnify computer screens and other objects.

Sadly, Fresnel died of tuberculosis at the age of 39, and he was mostly unknown during his lifetime.

This Fresnel lens was made around 1874 in Paris, France, and operated until 1995 in the east Cape Elizabeth Lighthouse tower at Two Lights. It is part of the collection at the Maine Maritime Museum in Bath.

Zac McDorr is the founder of the Bath Maine History Center on Facebook.You can reach him at zacmcdorr@gmail.com.