with an _opacous_ body, though never so thin. And this I have often try'd,
by pressing small _Globule_ of _Mercury_ between two smooth Plates of
Glass, whereby I have reduc'd that body to a much greater thinness then was
requisite to exhibit the colours with a transparent body.

Thirdly, there must be a considerable reflecting body adjacent to the under
or further side of the _lamina_ or _plate_: for this I always found, that
the greater that reflection was, the more vivid were the appearing colours.

From which Observations, is most evident, that the reflection from the
under or further side of the body is the principal cause of the production
of these colours; which, that it is so, and how it conduces to that effect,
I shall further explain in the following Figure, which is here described of
a very great thickness, as if it had been view'd through the _Microscope_;
and 'tis indeed much thicker than any _Microscope_ (I have yet us'd) has
been able to shew me those colour'd plates of Glass, or _Muscovie-glass_,
which I have not without much trouble view'd with it, for though I have
endeavoured to magnifie them as much as the Glasses were capable of, yet
are they so exceeding thin, that I have not hitherto been able positively
to determine their thickness. This Figure therefore I here represent, is
wholy _Hypothetical_.

Let ABCDHFE in the sixth Figure be a _frustum_ of _Muscovy-glass_, thinner
toward the end AE, and thicker towards DF. Let us first suppose the Ray
aghb coming from the Sun, of some remote luminous object to fall
_obliquely_ on the thinner plate BAE, part therefore is reflected back by
cghd, the first _Superficies_; whereby the perpendicular pulse ab is after
reflexion propagated by cd, cd, equally remote from each other with ab, ab,
so that ag + gc, or bh + hd are either of them equal to aa, as is also cc,
but the body BAE being transparent, a part of the light of this Ray is