progress it
inclines the Ray at one distance from the first Surface to be reflected
by the second, at another to be transmitted by it, and that by equal
Intervals for innumerable vicissitudes. And because the Ray is disposed
to Reflexion at the distances 1, 3, 5, 7, 9, &c. and to Transmission at
the distances 0, 2, 4, 6, 8, 10, &c. (for its transmission through the
first Surface, is at the distance 0, and it is transmitted through both
together, if their distance be infinitely little or much less than 1)
the disposition to be transmitted at the distances 2, 4, 6, 8, 10, &c.
is to be accounted a return of the same disposition which the Ray first
had at the distance 0, that is at its transmission through the first
refracting Surface. All which is the thing I would prove.

What kind of action or disposition this is; Whether it consists in a
circulating or a vibrating motion of the Ray, or of the Medium, or
something else, I do not here enquire. Those that are averse from
assenting to any new Discoveries, but such as they can explain by an
Hypothesis, may for the present suppose, that as Stones by falling upon
Water put the Water into an undulating Motion, and all Bodies by
percussion excite vibrations in the Air; so the Rays of Light, by
impinging on any refracting or reflecting Surface, excite vibrations in
the refracting or reflecting Medium or Substance, and by exciting them
agitate the solid parts of the refracting or reflecting Body, and by
agitating them cause the Body to grow warm or hot; that the vibrations
thus excited are propagated in the refracting or reflecting Medium or
Substance, much after the manner that vibrations are propagated in the
Air for causing Sound, and move faster than the Rays so as to overtake
them; and that when any Ray is in that part of the vibration which
conspires with its Motion, it easily breaks through a refracting
Surface, but when it is in the contrary part of the vibration which
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 ca