How can you know which pages in a medieval manuscript were the most popular? Kathryn Rudy of the University of St. Andrews reasoned that the finger smudges left by readers would be a good indicator:
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.