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 stonenb.... Scribes, illuminators, and scholars held such stones directly over manuscript pages as an <WSD>aid</WSD> in seeing what was being written, drawn, or read. 
Meaning of the word marked with <WSD> tags:  Something which helps; a material source of help.