Sentence:  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 <WSD>paperweight</WSD>. 
Meaning of the word marked with <WSD> tags:  A small, decorative, somewhat weighty object placed on one or more pieces of paper to keep them from fluttering away.