whatsoever. This have I also sometimes with pleasure observ'd even in
Muscles and Tendons. Further, if you take any glutinous substance and run
it exceedingly thin upon the surface of a smooth glass or a polisht
metaline body, you shall find the like effects produced: and in general,
wheresoever you meet with a transparent body thin enough, that is
terminated by reflecting bodies of differing refractions from it, there
will be a production of these pleasing and lovely colours.

Nor is it necessary, that the two _terminating_ Bodies should be both of
the same kind, as may appear by the _vitrified Laminae_ on _Steel_, _Lead_,
and other Metals, one surface of which _Laminae_ is contiguous to the
surface of the Metal, the other to that of the Air.

Nor is it necessary, that these colour'd _Laminae_ should be of an even
thickness, that is, should have their edges and middles of equal thickness,
as in a Looking-glass-plate, which circumstance is only requisite to make
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