As early as the 13th century, glasses were available to accommodate the reader. That is, if you did not loose them. In the first part of this series on medieval glasses, for example, a reader dropped them into the toilet. The images above illustrate a different kind of loss. They show an imprint of a pair of glasses, which, judging from the stain, had been stuck between the pages for quite some time. The reader likely closed his book with his glasses still inside, placed it back on the shelves, and forgot where he had left them. The trapped device became a stowaway, a time traveller that was released quite some time later. The imprint is a reminder of its journey - and of the fact that misplacing your glasses is of all times.
Pic: Utrecht, Universiteitsbibliotheek, MS 1003 (late 11th century), tweeted some time ago by @edvandervlist. Another imprint of medieval glasses is discussed and illustrated here.