the Earth's Orbit. Some inequalities of time may arise from the
Excentricities of the Orbs of the Satellites; but those cannot answer in
all the Satellites, and at all times to the Position and Distance of the
Earth from the Sun. The mean motions of _Jupiter_'s Satellites is also
swifter in his descent from his Aphelium to his Perihelium, than in his
ascent in the other half of his Orb. But this inequality has no respect
to the position of the Earth, and in the three interior Satellites is
insensible, as I find by computation from the Theory of their Gravity.
PROP. XII.
_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
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