_Those Superficies of transparent Bodies reflect the greatest quantity
of Light, which have the greatest refracting Power; that is, which
intercede Mediums that differ most in their refractive Densities. And in
the Confines of equally refracting Mediums there is no Reflexion._
The Analogy between Reflexion and Refraction will appear by considering,
that when Light passeth obliquely out of one Medium into another which
refracts from the perpendicular, the greater is the difference of their
refractive Density, the less Obliquity of Incidence is requisite to
cause a total Reflexion. For as the Sines are which measure the
Refraction, so is the Sine of Incidence at which the total Reflexion
begins, to the Radius of the Circle; and consequently that Angle of
Incidence is least where there is the greatest difference of the Sines.
Thus in the passing of Light out of Water into Air, where the Refraction
is measured by the Ratio of the Sines 3 to 4, the total Reflexion begins
when the Angle of Incidence is about 48 Degrees 35 Minutes. In passing
out of Glass into Air, where the Refraction is measured by the Ratio of
the Sines 20 to 31, the total Reflexion begins when the Angle of
Incidence is 40 Degrees 10 Minutes; and so in passing out of Crystal, or
more strongly refracting Mediums into Air, there is still a less
obliquity requisite to cause a total reflexion. Superficies therefore
which refract most do soonest reflect all the Light which is incident on
them, and so must be allowed most strongly reflexive.
But the truth of this Proposition will farther appear by observing, that
in the Superficies interceding two transparent Mediums, (such as are
Air, Water, Oil, common Glass, Crystal, metalline Glasses, Island
Glasses, white transparent Arsenick, Diamonds, &c.) the Reflexion is
stronger or weaker accordingly, as the Superficies hath a greater or
less refracting Power. For in the Confine of Air and Sal-gem 'tis
stronger than in the Confine of Air and Water, and still stronger in the
Confine of Air and common Glass or Crystal, and stronger in the Confine
of Air and a Diamond. If any of these, and such like transparent Solids,
be immerged in Water, its Reflexion becomes, much weaker than before;
and still weaker if they be immerged in the more strongly refracting
Liquors of well rectified Oil of Vitriol or Spirit of Turpentine. If
Water be distinguish'd into two parts by any imaginary Surface, the
Reflexion in the Confine of those two parts is none at all. In the
Confine of Water and Ice 'tis very little; in that of Water and Oil 'tis
something greater; in that of Water and Sal-gem still greater; and in
that of Water and Glass, or Crystal or other denser Substances still
greater, accordingly as those Mediums differ more or less in their
refracting Powers. Hence in the Confine of common Glass and Crystal,
there ought to be a weak Reflexion, and a stronger Reflexion in the
Confine of common and metalline Glass; though I have not yet tried
this. But in the Confine of two Glasses of equal density, there is not
any sensible Reflexion; as was shewn in the first Observation. And the
same may be understood of the Superficies interceding two Crystals, or
two Liquors, or any other Substances in which no Refraction is caused.
So then the reason why uniform pellucid Mediums (such as Water, Glass,
or Crystal,) have no sensible Reflexion but in their external
Superficies, where they are adjacent to other Mediums of a different
density, is because all their contiguous parts have one and the same
degree of density.