Light: A Radiant History from Creation to the Quantum Age by Bruce Watson

Author:Bruce Watson [Watson, Bruce]
Language: eng
Format: epub
ISBN: 9781620405611
Publisher: Bloomsbury Publishing
Published: 2016-02-02T05:00:00+00:00

CHAPTER 11

Lumière:

France’s Dazzling Century

The urchin of present-day Paris … is at once a national emblem and a disease. A disease that must be cured. How? By light.

—VICTOR HUGO, LES MISÉRABLES

Early in the nineteenth century, sailors on tall ships entering the North Atlantic knew they were bound for a dark and dangerous place—France. The revolution was long past, but stories lingered—of the guillotine, of riots and rage, of Napoleon, that scourge of all Europe. Paris, owing to its role in the Enlightenment, was coming to be called the City of Light, yet Paris itself was a scourge, a labyrinth of disease and decay, its air reeking of emptied chamber pots, its river yellowed by raw sewage. City of Light? Not in the warrens where most lived, certainly not in winter, when, as in Les Misérables, “the sky had become a grating, the day a cellar, and the sun a poor man at the door.”

The dangers of France were not confined to Paris. The nation’s rocky coastline had few lighthouses, each so dim that a ship was almost upon it before seeing its beam. Each year nearly two hundred ships ran aground off French shores. Still, sailors drifted toward France’s western flank. There the river Gironde led to the city of Bordeaux, whose wines the world demanded. As the coast loomed, the search began for the lighthouse—le phare—at the river’s mouth.

The Cordouan lighthouse was one of Europe’s oldest and tallest. Perched on an outcropping a day’s sail north of Spain, Cordouan was “the Versailles of the sea.” In regal French style, the ten-story tower featured a first-floor bedroom readied for the king, a second-floor chapel, and abundant statues throughout. The light, however, was no stronger than when it first shone back in the age of Descartes. Then it had been a bonfire, but in recent years, keepers had experimented with lamps and mirrors whose puny light had sailors begging France to make this phare worthy of its namesake, Alexandria’s Pharos. Napoleon established a lighthouse commission, but it rarely met. Complaints went unheeded. The Cordouan light remained a flickering candle. Then, in 1820, the world’s leading expert on light began studying the problem.

The problem was reflection, Augustin-Jean Fresnel concluded. Even the largest parabolic mirror placed behind the strongest lamp reflected less than half the ambient light. Applying his elegant integrals and his “taste for exactitude,” Fresnel calculated how to capture light by refraction rather than reflection. Within months, glass factories in Paris were turning out prisms three inches thick and twenty inches long. Following Fresnel’s design of “lenses by steps,” glassmakers assembled polygonal plates two feet across, each ribbed with crystalline circles that focused light from all directions. By April 1821, Fresnel’s lens, a twelve-foot tall glass beehive, was ready for testing. Placed atop the Royal Observatory south of Paris’s Luxembourg Gardens, the lens shot a beam over the Seine, past Abbot Suger’s Saint-Denis, and beyond the hills of Montmartre. Within a year, several hundred pounds of prisms and glass plates were shipped around the cliffs of Brittany and landed at the Cordouan light.

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