_Every Ray of Light in its passage through any refracting Surface is put
into a certain transient Constitution or State, which in the progress of
the Ray returns at equal Intervals, and disposes the Ray at every return
to be easily transmitted through the next refracting Surface, and
between the returns to be easily reflected by it._
This is manifest by the 5th, 9th, 12th, and 15th Observations. For by
those Observations it appears, that one and the same sort of Rays at
equal Angles of Incidence on any thin transparent Plate, is alternately
reflected and transmitted for many Successions accordingly as the
thickness of the Plate increases in arithmetical Progression of the
Numbers, 0, 1, 2, 3, 4, 5, 6, 7, 8, &c. so that if the first Reflexion
(that which makes the first or innermost of the Rings of Colours there
described) be made at the thickness 1, the Rays shall be transmitted at
the thicknesses 0, 2, 4, 6, 8, 10, 12, &c. and thereby make the central
Spot and Rings of Light, which appear by transmission, and be reflected
at the thickness 1, 3, 5, 7, 9, 11, &c. and thereby make the Rings which
appear by Reflexion. And this alternate Reflexion and Transmission, as I
gather by the 24th Observation, continues for above an hundred
vicissitudes, and by the Observations in the next part of this Book, for
many thousands, being propagated from one Surface of a Glass Plate to
the other, though the thickness of the Plate be a quarter of an Inch or
above: So that this alternation seems to be propagated from every
refracting Surface to all distances without end or limitation.
This alternate Reflexion and Refraction depends on both the Surfaces of
every thin Plate, because it depends on their distance. By the 21st
Observation, if either Surface of a thin Plate of _Muscovy_ Glass be
wetted, the Colours caused by the alternate Reflexion and Refraction
grow faint, and therefore it depends on them both.
It is therefore perform'd at the second Surface; for if it were
perform'd at the first, before the Rays arrive at the second, it would
not depend on the second.
It is also influenced by some action or disposition, propagated from the
first to the second, because otherwise at the second it would not depend
on the first. And this action or disposition, in its propagation,
intermits and returns by equal Intervals, because in all its 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.