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
Light: All which compared together evince and rarify both this and the
last Proposition.
PROP. XV.
_In any one and the same sort of Rays, emerging in any Angle out of any
refracting Surface into one and the same Medium, the Interval of the
following Fits of easy Reflexion and Transmission are either accurately
or very nearly, as the Rectangle of the Secant of the Angle of
Refraction, and of the Secant of another Angle, whose Sine is the first
of 106 arithmetical mean Proportionals, between the Sines of Incidence
and Refraction, counted from the Sine of Refraction._
This is manifest by the 7th and 19th Observations.
PROP. XVI.
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