我要---單片眼鏡monocle---的資料

你說的應該是 monocle
[image]http://upload.wikimedia.org/wikipedia/en/9/90/Chamberlain.jpg[/image]

根據 Wikipedia 對於「[href=http://en.wikipedia.org/wiki/Monocle]單片眼鏡[/href]」的說明：
[quote]Joseph Chamberlain wearing a monocleA monocle is a type of corrective lens, descended from the Quizzing glasses of the 1700s, used to correct the vision in only one eye. It consists of a circular lens, generally with a wire ring around the circumference that can be attached to a string. The other end of the string is then connected to the wearer's clothing to avoid losing the monocle.[/quote]

此外還可以看看其他條目，如"history of lensmaking"、"glasses"、"magnifier"

你指的「中世紀的人」應該是外國人吧?!（在中國只有邊陲國家才有國王）若是外國的話，的確，透鏡這樣的科技不似今日普及的原因在於過去對於知識流傳僅限於少部份的人：王公貴族及僧侶。他們用的該算凸透鏡。

更早之前有能力閱讀的人應該不多，真視力不好的話，可以叫視力好的書僮／家僕／隨侍念給他聽。

參考資料 Wikipedia & my own inference