Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Pĭla Vitrea

A glass globe filled with water for the purpose of being placed between a person and the object he is contemplating, in order to magnify the object and render it clearer to the view—a custom still adopted in wood-engraving. It would also appear from a passage of Seneca (Q. N. i. 6) that this contrivance was sometimes employed by the ancients to assist an imperfect or failing sight, in the place of our spectacles. It must, however, be remembered that the ancients, who employed a numerous class of well-educated slaves in the character of readers, secretaries, and amanuenses, did not stand so much in need of an artificial aid to the eyesight as we moderns do.