heir parts be subtilly divided, (as Metals,
by being dissolved in acid Menstruums, &c.) become perfectly
transparent. And you may also remember, that in the eighth Observation
there was no sensible reflexion at the Superficies of the
Object-glasses, where they were very near one another, though they did
not absolutely touch. And in the 17th Observation the Reflexion of the
Water-bubble where it became thinnest was almost insensible, so as to
cause very black Spots to appear on the top of the Bubble, by the want
of reflected Light.

On these grounds I perceive it is that Water, Salt, Glass, Stones, and
such like Substances, are transparent. For, upon divers Considerations,
they seem to be as full of Pores or Interstices between their parts as
other Bodies are, but yet their Parts and Interstices to be too small to
cause Reflexions in their common Surfaces.

PROP. V.

_The transparent parts of Bodies, according to their several sizes,
reflect Rays of one Colour, and transmit those of another, on the same
grounds that thin Plates or Bubbles do reflect or transmit those Rays.
And this I take to be the ground of all their Colours._

For if a thinn'd or plated Body, which being of an even thickness,
appears all over of one uniform Colour, should be slit into Threads, or
broken into Fragments, of the same thickness with the Plate; I see no
reason why every Thread or Fragment should not keep its Colour, and by
consequence why a heap of those Threads or Fragments should not
constitute a Mass or Powder of the same Colour, which the Plate
exhibited before it was broken. And the parts of all natural Bodies
being like so many Fragments of a Plate, must on the same grounds
exhibit the same Colours.

Now, that they do so will appear by the affinity of their Properties.
The finely colour'd Feathers of some Birds, and particularly those of
Peacocks Tails, do, in the very same part of the Feather, appear of
several Colours in several Positions of the Eye, after the very same
manner that thin Plates were found to do in the 7th and 19th
Observations, and therefore their Colours arise from the thinness of the
transparent parts of the Feathers; that is, from the slenderness of the
very fine Hairs, or _Capillamenta_, which grow out of the sides of the
grosser lateral Branches or Fibres of those Feathers. And to the same
purpose it is, that the Webs of some Spiders, by being spun very fine,
have appeared colour'd, as some have observ'd