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 <WSD>called</WSD> a reading stone, essentially what today we might term a frameless magnifying glass or plain glass paperweight. 
Meaning of the word marked with <WSD> tags:  Of a person, to have as one's name; of a thing, to have as its name.