ll
its_ Fits of easy Reflexion, _and those of its disposition to be
transmitted its_ Fits of easy Transmission, _and the space it passes
between every return and the next return, the_ Interval of its Fits.

PROP. XIII.

_The reason why the Surfaces of all thick transparent Bodies reflect
part of the Light incident on them, and refract the rest, is, that some
Rays at their Incidence are in Fits of easy Reflexion, and others in
Fits of easy Transmission._

This may be gather'd from the 24th Observation, where the Light
reflected by thin Plates of Air and Glass, which to the naked Eye
appear'd evenly white all over the Plate, did through a Prism appear
waved with many Successions of Light and Darkness made by alternate Fits
of easy Reflexion and easy Transmission, the Prism severing and
distinguishing the Waves of which the white reflected Light was
composed, as was explain'd above.

And hence Light is in Fits of easy Reflexion and easy Transmission,
before its Incidence on transparent Bodies. And probably it is put into
such fits at its first emission from luminous Bodies, and continues in
them during all its progress. For these Fits are of a lasting nature, as
will appear by the next part of this Book.

In this Proposition I suppose the transparent Bodies to be thick;
because if the thickness of the Body be much less than the Interval of
the Fits of easy Reflexion and Transmission of the Rays, the Body loseth
its reflecting power. For if the Rays, which at their entering into the
Body are put into Fits of easy Transmission, arrive at the farthest
Surface of the Body before they be out of those Fits, they must be
transmitted. And this is the reason why Bubbles of Water lose their
reflecting power when they grow very thin; and why all opake Bodies,
when reduced into very small parts, become transparent.

PROP. XIV.

_Those Surfaces of transparent Bodies, which if the Ray be in a Fit of
Refraction do refract it most strongly, if the Ray be in a Fit of
Reflexion do reflect it most easily._

For we shewed above, in _Prop._ 8. that the cause of Reflexion is not
the impinging of Light on the solid impervious parts of Bodies, but some
other power by which those solid parts act on Light at a distance. We
shewed also in _Prop._ 9. that Bodies reflect and refract Light by one
and the same power, variously exercised in various circumstances; and in
_Prop._ 1. that the most strongly refracting Surfaces reflect the most