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,.... Scribes, illuminators, and scholars held such stones <WSD>directly</WSD> over manuscript pages as an aid in seeing what was being written, drawn, or read. 
Meaning of the word marked with <WSD> tags:  Exactly; just.