Sentence:  The ability of a <WSD>segment</WSD> of a glass sphere to magnify whatever is placed before it was known around the year 1000, when the spherical <WSD>segment</WSD> was called a reading stone,.... Scribes, illuminators, and scholars held such stones directly over manuscript pages as an aid in seeing what was being written, drawn, or read. 
Meaning of the word marked with <WSD> tags:  One of the parts into which any body naturally separates or is divided; a part divided or cut off; a section; a portion.