_PART III._
_Of the permanent Colours of natural Bodies, and the Analogy between
them and the Colours of thin transparent Plates._
I am now come to another part of this Design, which is to consider how
the Ph nomena of thin transparent Plates stand related to those of all
other natural Bodies. Of these Bodies I have already told you that they
appear of divers Colours, accordingly as they are disposed to reflect
most copiously the Rays originally endued with those Colours. But their
Constitutions, whereby they reflect some Rays more copiously than
others, remain to be discover'd; and these I shall endeavour to manifest
in the following Propositions.
PROP. I.
_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.
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