Glass is lighter, stiffer, less costly and easier to work than speculum metal.
The use of the convex lens, which is given as a great secret, in place of the concave speculum of the first edition, is not so clearly described as by Barbaro; the addition of the concave speculum is proposed for making the images larger and clearer, and also for making them erect, but no details are given.
Sabatier's theory as to the nature of these documents was, in brief, that the Speculum perfectionis was the first of all the Lives of the saint, written in 1227 by Br.
Leo; on the other hand, Thomas of Celano's two Lives are free from the "tendencies" ascribed to them by Sabatier, and that of 1248 was written with the collaboration of Leo and the other companions; thus the best sources of information are those portions of the Speculum that can with certainty be carried back to Br.
Rutherfurd introduced into common use the reflection grating, finding that speculum metal was less trying than glass to the diamond point, upon the permanence of which so much depends.