the Plate appear all of the same colour; but they may resemble a _Lens_,
that is, have their middles thicker then their edges; or else a _double
concave_, that is, be thinner in the middle then at the edges; in both
which cases there will be various coloured rings or lines, with differing
consecutions or orders of Colours; the order of the first from the middle
outwards being Red, Yellow, Green, Blew, &c. And the latter quite contrary.

But further, it is altogether necessary, that the Plate, in the places
where the Colours appear, should be of a determinate thickness: First, It
must not be more then such a thickness, for when the Plate is increased to
such a thickness, the Colours cease; and besides, I have seen in a thin
piece of _Muscovy-glass_, where the two ends of two Plates, which appearing
both single, exhibited two distinct and differing Colours; but in that
place where they were united, and constituted one double Plate (as I may
call it) they appeared transparent and colourless. Nor, Secondly, may the
Plates be _thinner_ then such a determinate _cize_; for we alwayes find,
that the very outmost Rim of these flaws is terminated in a white and
colourless Ring.

Further, in this Production of Colours there is no need of a determinate
Light of such a bigness and no more, nor of a determinate position of that
Light, that it should be on this side, and not on that side; nor of a
terminating shadow, as in the Prisme, and Rainbow, or Water-ball: for we
find, that the Light in the open Air, either in or out of the Sun-beams,
and within a Room, either from one or many Windows, produces much the same
effect: only where the Light is brightest, there the Colours are most
_vivid_. So does the light of a Candle, collected by a Glass-ball. And
further, it is all one whatever side of the coloured Rings be towards the
light; for the whole Ring keeps its proper Colours from the middle outwards
in the same order as I before related, without varying at all, upon