_The parts of Bodies on which their Colours depend, are denser than the
Medium which pervades their Interstices._
This will appear by considering, that the Colour of a Body depends not
only on the Rays which are incident perpendicularly on its parts, but on
those also which are incident at all other Angles. And that according to
the 7th Observation, a very little variation of obliquity will change
the reflected Colour, where the thin Body or small Particles is rarer
than the ambient Medium, insomuch that such a small Particle will at
diversly oblique Incidences reflect all sorts of Colours, in so great a
variety that the Colour resulting from them all, confusedly reflected
from a heap of such Particles, must rather be a white or grey than any
other Colour, or at best it must be but a very imperfect and dirty
Colour. Whereas if the thin Body or small Particle be much denser than
the ambient Medium, the Colours, according to the 19th Observation, are
so little changed by the variation of obliquity, that the Rays which
are reflected least obliquely may predominate over the rest, so much as
to cause a heap of such Particles to appear very intensely of their
Colour.
It conduces also something to the confirmation of this Proposition,
that, according to the 22d Observation, the Colours exhibited by the
denser thin Body within the rarer, are more brisk than those exhibited
by the rarer within the denser.