Moscovy-glass, or Lapis specularis, is a Body that seems to have as many Curiosities in its Fabrick as any common Mineral I have met with:  for first, It is transparent to a great thickness:  Next, it is compounded of an infinite number of thin flakes joyned or generated one upon another so close & smooth, as with many hundreds of them to make one smooth and thin Plate of a transparent flexible substance, which with care and diligence may be flit into pieces so exceedingly thin as to be hardly perceivable by the eye, and yet even those, which I have thought the thinnest, I have with a good Microscope found to be made up of many other Plates, yet thinner; and it is probable, that, were our Microscopes much better, we might much further discover its divisibility.  Nor are these flakes only regular as to the smoothness of their Surfaces, but thirdly, In many Plates they may be perceived to be terminated naturally with edges of the figure of a Rhomboeid.  This Figure is much more conspicuous in our English talk, much whereof is found in the Lead Mines, and is commonly called Spar, and Kauck, which is of the same kind of substance with the Selenitis, but is seldom found in so large flakes as that is, nor is it altogether so tuff, but is much more clear and transparent, and much more curiously shaped, and yet may be cleft and flak’d like the other Selenitis.  But fourthly, this stone has a property, which in respect of the Microscope, is more notable, and that is, that it exhibits several appearances of Colours, both to the naked Eye, but much more conspicuously to the Microscope; for the exhibiting of which, I took a piece of Muscovy-glass, and splitting or cleaving it into thin Plates, I found that up and down in several parts of them I could plainly perceive several white specks or flaws, and others diversly coloured with all the Colours of the Rainbow; and with the Microscope I could perceive, that these Colours were ranged in rings that incompassed the white speck or flaw, and were round or irregular, according to the shape of the spot which they terminated; and the position of Colours, in respect of one another, was the very same as in the Rainbow.  The consecution of those Colours from the middle of the spot outward being Blew, Purple, Scarlet, Yellow, Green; Blew, Purple, Scarlet, and so onwards, sometimes half a score times repeated, that is, there appeared six, seven, eight, nine or ten several coloured rings or lines, each incircling the other, in the same manner as I have often seen a very vivid Rainbow to have four or five several Rings of Colours, that is, accounting all the Gradations between Red and Blew for one:  But the order of the Colours in these Rings was quite contrary to the primary or innermost Rainbow, and the same with those of the secondary or outermost Rainbow; these coloured Lines or Irises, as I may so call them, were some of them much brighter