Glass is lighter, stiffer, less costly and easier to work than speculum metal.
Francois and Speculum perfectionis, and Lempp's Frere Elie de Cortone.
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.
Thus in the Speculum Naturale of Vincent of Beauvais (c. 1250) it is said that there are four spirits - mercury, sulphur, arsenic and sal ammoniac - and six bodies - gold, silver, copper, tin, lead and iron.
Simon's history, in its original form, is lost; but large sections of it have been preserved in Vincent of Beauvais's Speculum historiale, where nineteen chapters are expressly said to be ex libello fratris Simonis, or entitled frater Simon.