To what uses coloured flints and crystals are appointed, is not here to be treated of; one use excepted, which I set down for the eyes, which are weakened by too much watching, the heat of fire and smoak; see thou have a waxen mold circularly round, of the bigness of a dish or trencher; (the Optiques are wont to call such _lentes_) to which, put the best clay well mixed with hair: anoint the waxen type with oyl, and exactly apply the best prepared earth of crucibles (and durable in the fire) the thickness of a finger; which being dryed, perforate in some part, that the wax being melted by the fire, may flow forth: afterward burn the mold in an earthen furnace; being burnt, fill it with prepared glass, and place it in a wind furnace till the glass melt; which at length being cooled, take off the type by attrition, and there shalt thou have the crystal resembling the form of the type; which afterward thou must make and polish like spectacles in an iron dish on both sides; and take it out with a strong iron wier, and thou shalt have a good crystalline _lent_ for a small price, which otherwise is scarce made of crystal of so great a bigness. And if thou wilt, thou maist colour the glass green, very pleasant to the sight, and fit a foot to it for greater benefit. And the glass doth not only serve for the Multiplication of light in the night time, that you may see a thing a far off in a chamber, but also for the fixing and calcining minerals by the Sun-beams, and melting of Metals, and multiplying of Pictures, like an hollow glass, and also for other uses it may be compared with an hollow looking-glass, which doth the same of an equal bigness with the hollow glass; nor is there any other difference of them but reflexion. This glass-instrument is made likewise another way, and by less cost and labor, if it be of a polisht looking-glass, if two great orbes are cut out with a diamond, and if they are somewhat softned with Fire, and are left there so long in the heat, until they shall stick like wax very close to the stone, which done, let them be cooled again, which afterward taken out, will represent the form of an hollow glass; to which, it behoves to fix a leaf on the convex part. And the glasses do the same that an hollow metallick looking-glass doth, the reflexion excepted, which is not so strong as of the hollow glass: And although the glasses are sooner broke, yet they are very fit for the making of the following Instrument.