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; thes