"reading stone" meaning in English

{
  "forms": [
    {
      "form": "reading stones",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "reading stone (plural reading stones)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "2013 September-October, Henry Petroski, “The Evolution of Eyeglasses”, in American Scientist",
          "text": "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, essentially what today we might term a frameless magnifying glass or plain glass paperweight. Scribes, illuminators, and scholars held such stones directly over manuscript pages as an aid in seeing what was being written, drawn, or read.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "A hemisphere of glass used as a magnifier when placed with its flat side against a surface with text."
      ],
      "id": "en-reading_stone-en-noun-Xn9Wkr-g",
      "links": [
        [
          "hemisphere",
          "hemisphere"
        ],
        [
          "glass",
          "glass"
        ],
        [
          "magnifier",
          "magnifier"
        ]
      ]
    }
  ],
  "word": "reading stone"
}

{
  "forms": [
    {
      "form": "reading stones",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "reading stone (plural reading stones)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations"
      ],
      "examples": [
        {
          "ref": "2013 September-October, Henry Petroski, “The Evolution of Eyeglasses”, in American Scientist",
          "text": "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, essentially what today we might term a frameless magnifying glass or plain glass paperweight. Scribes, illuminators, and scholars held such stones directly over manuscript pages as an aid in seeing what was being written, drawn, or read.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "A hemisphere of glass used as a magnifier when placed with its flat side against a surface with text."
      ],
      "links": [
        [
          "hemisphere",
          "hemisphere"
        ],
        [
          "glass",
          "glass"
        ],
        [
          "magnifier",
          "magnifier"
        ]
      ]
    }
  ],
  "word": "reading stone"
}