Bodies to be
so disposed amongst themselves, that the Intervals or empty Spaces
between them may be equal in magnitude to them all; and that these
Particles may be composed of other Particles much smaller, which have as
much empty Space between them as equals all the Magnitudes of these
smaller Particles: And that in like manner these smaller Particles are
again composed of others much smaller, all which together are equal to
all the Pores or empty Spaces between them; and so on perpetually till
you come to solid Particles, such as have no Pores or empty Spaces
within them: And if in any gross Body there be, for instance, three such
degrees of Particles, the least of which are solid; this Body will have
seven times more Pores than solid Parts. But if there be four such
degrees of Particles, the least of which are solid, the Body will have
fifteen times more Pores than solid Parts. If there be five degrees, the
Body will have one and thirty times more Pores than solid Parts. If six
degrees, the Body will have sixty and three times more Pores than solid
Parts. And so on perpetually. And there are other ways of conceiving how
Bodies may be exceeding porous. But what is really their inward Frame is
not yet known to us.

PROP. IX.

_Bodies reflect and refract Light by one and the same power, variously
exercised in various Circumstances._

This appears by several Considerations. First, Because when Light goes
out of Glass into Air, as obliquely as it can possibly do. If its
Incidence be made still more oblique, it becomes totally reflected. For
the power of the Glass after it has refracted the Light as obliquely as
is possible, if the Incidence be still made more oblique, becomes too
strong to let any of its Rays go through, and by consequence causes
total Reflexions. Secondly, Because Light is alternately reflected and
transmitted by thin Plates of Glass for many Successions, accordingly as
the thickness of the Plate increases in an arithmetical Progression. For
here the thickness of the Glass determines whether that Power by which
Glass acts upon Light shall cause it to be reflected, or suffer it to
be transmitted. And, Thirdly, because those Surfaces of transparent
Bodies which have the greatest refracting power, reflect the greatest
quantity of Light, as was shewn in the first Proposition.

PROP. X.

_If Light be swifter in Bodies than in Vacuo, in the proportion of the
Sines which measure the Refraction of the