en
the thickness of a Plate requisite to produce any Colour, depends only
on the density of the Plate, and not on that of the ambient Medium. And
hence, by the 10th and 16th Observations, may be known the thickness
which Bubbles of Water, or Plates of _Muscovy_ Glass, or other
Substances, have at any Colour produced by them.

_Obs._ 22. A thin transparent Body, which is denser than its ambient
Medium, exhibits more brisk and vivid Colours than that which is so much
rarer; as I have particularly observed in the Air and Glass. For blowing
Glass very thin at a Lamp Furnace, those Plates encompassed with Air did
exhibit Colours much more vivid than those of Air made thin between two
Glasses.

_Obs._ 23. Comparing the quantity of Light reflected from the several
Rings, I found that it was most copious from the first or inmost, and in
the exterior Rings became gradually less and less. Also the whiteness of
the first Ring was stronger than that reflected from those parts of the
thin Medium or Plate which were without the Rings; as I could manifestly
perceive by viewing at a distance the Rings made by the two
Object-glasses; or by comparing two Bubbles of Water blown at distant
Times, in the first of which the Whiteness appear'd, which succeeded all
the Colours, and in the other, the Whiteness which preceded them all.

_Obs._ 24. When the two Object-glasses were lay'd upon one another, so
as to make the Rings of the Colours appear, though with my naked Eye I
could not discern above eight or nine of those Rings, yet by viewing
them through a Prism I have seen a far greater Multitude, insomuch that
I could number more than forty, besides many others, that were so very
small and close together, that I could not keep my Eye steady on them
severally so as to number them, but by their Extent I have sometimes
estimated them to be more than an hundred. And I believe the Experiment
may be improved to the Discovery of far greater Numbers. For they seem
to be really unlimited, though visible only so far as they can be
separated by the Refraction of the Prism, as I shall hereafter explain.

[Illustration: FIG. 5.]

But it was but one side of these Rings, namely, that towards which the
Refraction was made, which by that Refraction was render'd distinct, and
the other side became more confused than when view'd by the naked Eye,
insomuch that there I could not discern above one or two, and sometimes
none of those Rings, of which I could