And because these bodies were so small, that I could not well come to make
Experiments and Examinations of them, I provided me several small _stiriae_
of Crystals or Diamants, found in great quantities in _Cornwall_ and are
therefore commonly called _Cornish Diamants_: these being very _pellucid_,
and growing in a hollow cavity of a Rock (as I have been several times
informed by those that have observ'd them) much after the same manner as
these do in the Flint, and having besides their outward surface very
regularly shap'd, retaining very near the same Figures with some of those I
observ'd in the other, became a convenient help to me for the Examination
of the proprieties of those kinds of bodies.
And first for the Reflections, in these I found it very observable, That
the brightest reflections of light proceeded from within the _pellucid_
body; that is, that the Rays admitted through the _pellucid_ substance in
their getting out on the opposite side, were by the contiguous and strong
reflecting surface of the Air very vividly reflected, so that more Rays
were reflected to the eye by this surface, though the Ray in entring and
getting out of the Crystal had suffer'd a double refraction, than there
were from the outward surface of the Glass where the Ray had suffer'd no
reflection at all.
And that this was the surface of the Air that gave so vivid a
_re-percussion_ I try'd by this means I sunk half of a _stiria_ in Water,
so that only Water was contiguous to the under surface, and then the
internal reflection was so exceedingly faint, that it was scarce
discernable.
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