Sentence:  The ability of a segment of a <WSD>glass</WSD> 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 <WSD>glass</WSD> paperweight. 
Meaning of the word marked with <WSD> tags:  An amorphous solid, often transparent substance made by melting sand with a mixture of soda, potash and lime.