It is my belief the loupe or magnifying lens was placed in front of their master eye by a brass support hung around their neck or perhaps from their head. By then, lenses had existed for a while. That support concept is similar to lenses mounted on a brass ring, placed an inch or two above a chart, that a sea captain could use when reviewing charts. Any support to an eye loupe magnifier would certainly have allowed any master/guild craftsman to use both hands.
-Roger Bacon described the properties of a magnifying glass in 13th-century England.
-Eyeglasses were developed in 13th-century Italy.
-Though contact lenses seem to be a recent phenomenon, the famous Italian architect, mathematician and inventor Leonardo da Vinci (1452-1519) produced the first known sketches (in 1508) that suggested the optics of the human eye could be altered by placing the cornea directly in contact with water.