In his Aparokṣānubhūti verse 81, Śaṅkarācārya says: sūkṣmatve sarvavastūnāṃ sthūlatvaṃ copanetrataḥ. Here, some modern interpreters have interpreted the word upanetra as referring to eye-glasses or some lens that enlarges the view. I am just wondering about the history of lenses and eye-glasses around the time of Śaṅkara. An allied question will be the date and authorship of Aparokṣānubhūti. Any information and references are appreciated.