In fact, this final detail is a mistake, an anachronism. Hugh died acouple of decades too early to have benefitted from this invention:two magnifying glasses bound together by a rivet in their handles.
The image of Hugh of Saint-Cher wearing glasses in the fresco at the former convent of San Niccolò, Chapter Room, Treviso, is an anachronism as their invention was decades following his death in 1263.