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.

   1 more example

   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.

   2013 September-October, Henry Petroski, “The Evolution of Eyeglasses”, in American Scientist