Optics and the Fresnel Revolution
A tower is only as useful as the light it carries, and until the nineteenth century that light was weak. Coal fires gave way to banks of oil lamps fitted with parabolic reflectors, an improvement that still wasted most of the light that left the flame.
The transformation came from the French physicist Augustin-Jean Fresnel, who in the 1820s designed a lens built from concentric rings of prisms. Rather than a single thick and impractically heavy piece of glass, the Fresnel lens gathered light from a wide angle and bent it into a narrow horizontal beam, using a fraction of the material. Installed at the Cordouan lighthouse in 1823, the design increased effective range dramatically and was adopted internationally within decades.
A first-order Fresnel lens stood several metres tall and weighed several tonnes. Fresnel lenses were graded by order, from massive first-order assemblies several metres tall used on major landfall lights down to compact sixth-order lenses for harbour work. Rotating the lens, at first on chariot wheels and later floating on a bath of mercury, gave each station a distinct flash pattern, or characteristic, so that a mariner could identify which light they were seeing rather than merely that a light existed.
That shift, from a visible glow to an identifiable signal, is what turned lighthouses into a genuine navigational network rather than a scattering of warnings.