'Optics were likewise a branch of study which suited the natural genius of the Arabian philosophers, and which they pursued with incredible delight. This science was a part of the Aristotelic philosophy which, as I have before observed, they refined and filled with a thousand extravagances. Hence our strange knight's Mirror of Glass, prepared on the most profound principles of art, and endued with preternatural qualities (ll. 225-234, 132-141).
'Alcen, or Alhazen, mentioned in l. 232, an Arabic philosopher, wrote seven books of perspective, and flourished about the eleventh century. Vitellio, formed on the same school, was likewise an eminent mathematician of the middle ages, and wrote ten books on Perspective. The Roman Mirror here mentioned by Chaucer, as similar to this of the strange knight, is thus described by Gower:—
"Whan Rome stood in noble plite,
Virgile, which was tho parfite,
A mirrour made of his clergye [by his skill],
And sette it in the townes ye [eye, sight]
Of marbre on a piller withoute,
That they, by thritty mile aboute,
By day and eek also by nighte
In that mirrour beholde mighte