into many small Bubbles, either alone in the form of Froth, or by
shaking it together with Oil of Turpentine, or Oil Olive, or with some
other convenient Liquor, with which it will not perfectly incorporate.
And to the increase of the opacity of these Bodies, it conduces
something, that by the 23d Observation the Reflexions of very thin
transparent Substances are considerably stronger than those made by the
same Substances of a greater thickness.
PROP. IV.
_The Parts of Bodies and their Interstices must not be less than of some
definite bigness, to render them opake and colour'd._
For the opakest Bodies, if their 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
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