"Saunière was holding it when we found him," Fache said, leaving Langdon and moving several yards to a portable table covered with investigation tools, cables, and assorted electronic gear. "As I told you," he said, rummaging around the table, "we have touched nothing. Are you familiar with this kind of pen?"

Langdon knelt down farther to see the pen's label.

STYLO DE LUMIERE NOIRE.

He glanced up in surprise.

The black-light pen or watermark stylus was a specialized felt-tipped marker originally designed by museums, restorers, and forgery police to place invisible marks on items. The stylus wrote in a noncorrosive, alcohol-based fluorescent ink that was visible only under black light. Nowadays, museum maintenance staffs carried these markers on their daily rounds to place invisible "tick marks" on the frames of paintings that needed restoration.