Also the outward part of the looking-glasses (convex) may be smoothed, which represents the species short, and spreads the dispersed rays: but the inward part (hollow) gathers and multiplies, and puts forth or exposeth the Image. Let these things suffice concerning the melting of looking glasses, & polishing requisites, for the collection of the Sun beams, and although from the aforesaid mixture other kinds of looking-glasses might be made representing wonderful shapes and several excellent things, as Cylindrick, Pyramidal, Parabolick, _&c._ they are omitted as impertinent to this place, yet I could shew a way to make them, because I have undergone no small labors and charges in the searching of their preparation and use, if it were necessary. But of all looking-glasses that is most useful whose preparation we have shewn, whose diameter is at least two or three spans, if thou wilt perform any special thing; although it be but of one or two spans, yet it gathers abundance of beams, so that thou maist melt tin and lead with it, if it be well shaped: yet the larger are the better. Nor ought they to be too deep, that they may cast their beams the further, and better perform their actions or functions, let them have the twentyeth or thirtyeth part of the sphere (the section being exactly observed) which is the foundation of the Art. _Of Artificial Gems, and Metallick Glasses._ As for metallick glasses pertaining to Alchymy, and much conducing to the perfection of metals, and esteemed by the Ancient Philosophers, I would not omit to say somewhat in this place, because they are easily made by this furnace.