reading stone

reading stone (plural reading stones)

1. A hemisphere of glass used as a magnifier when placed with its flat side against a surface with text.
   • 2013 September-October, Henry Petroski, “The Evolution of Eyeglasses”, in American Scientist:

     The ability of a segment of a glass sphere to magnify whatever is placed before it was known around the year 1000, when the spherical segment was called a reading stone, essentially what today we might term a frameless magnifying glass or plain glass paperweight. Scribes, illuminators, and scholars held such stones directly over manuscript pages as an aid in seeing what was being written, drawn, or read.

• reading stone on Wikipedia.Wikipedia