On a recent visit to the Notre-Dame de Paris, the 12th-century cathedral was bustling with a steady flow of visitors who had come inside the Gothic monument from a wintery afternoon. It seemed more packed than it had been before its closure in 2019, after the collapse of its iconic spire and roof in a horrific blaze. But the line to get in moved quickly, and once under its vaulted ceiling, the sheer size of the structure left room to linger. I had gone to see the stained-glass windows designed by Eugène-Emmanuel Viollet-le-Duc in the 19th century, adorning six of the south-side chapels along the nave—before they are replaced.