In well-thumbed old books, certain pages are more thumbed—and therefore dirtier—than others. What do those pages say about reading habits? Using a device that measures the optical density of a reflective surface, Kathryn Rudy of the University of St. Andrews in Scotland asked this question of fifteenth-century personal devotional prayer books—Books of Hours—from the Netherlands. Hypothesizing that grime in the lower corners of pages would be roughly equivalent to time spent reading the page, Rudy took readings “from the juicy dirt at the bottom of the page,” she says. The most popular passages in these books tended to be prayers related to indulgences (time off in purgatory for forgiven sins) and health benefits, such as protection from plague or St. Anthony’s fire. Self-interest was the most common theme. In one manuscript that had been enhanced with custom illuminations, the owner primarily looked at pictures—in particular one that depicted the owner himself. “He really loved that image,” says Rudy.—Samir S. Patel