impedes its Motion, it is easily reflected; and, by consequence, that
every Ray is successively disposed to be easily reflected, or easily
transmitted, by every vibration which overtakes it. But whether this
Hypothesis be true or false I do not here consider. I content my self
with the bare Discovery, that the Rays of Light are by some cause or
other alternately disposed to be reflected or refracted for many
vicissitudes.
DEFINITION.
_The returns of the disposition of any Ray to be reflected I will call
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
160