pentine.| 25 to 17 | 1'1626 | 0'874 | 13222
Amber. | 14 to 9 | 1'42 | 1'04 | 13654
A Diamond. | 100 to 41 | 4'949 | 3'4 | 14556
---------------------+----------------+----------------+----------+-----------

The Refraction of the Air in this Table is determin'd by that of the
Atmosphere observed by Astronomers. For, if Light pass through many
refracting Substances or Mediums gradually denser and denser, and
terminated with parallel Surfaces, the Sum of all the Refractions will
be equal to the single Refraction which it would have suffer'd in
passing immediately out of the first Medium into the last. And this
holds true, though the Number of the refracting Substances be increased
to Infinity, and the Distances from one another as much decreased, so
that the Light may be refracted in every Point of its Passage, and by
continual Refractions bent into a Curve-Line. And therefore the whole
Refraction of Light in passing through the Atmosphere from the highest
and rarest Part thereof down to the lowest and densest Part, must be
equal to the Refraction which it would suffer in passing at like
Obliquity out of a Vacuum immediately into Air of equal Density with
that in the lowest Part of the Atmosphere.

Now, although a Pseudo-Topaz, a Selenitis, Rock Crystal, Island Crystal,
Vulgar Glass (that is, Sand melted together) and Glass of Antimony,
which are terrestrial stony alcalizate Concretes, and Air which probably
arises from such Substances by Fermentation, be Substances very
differing from one another in Density, yet by this Table, they have
their refractive Powers almost in the same Proportion to one another as
their Densities are, excepting that the Refraction of that strange
Substance, Island Crystal is a little bigger than the rest. And
particularly Air, which is 3500 Times rarer than the Pseudo-Topaz, and
4400 Times rarer than Glass of Antimony, and 2000 Times rarer than the
Selenitis, Glass vulgar, or Crystal of the Rock, has notwithstanding its
rarity the same refractive Power in respect of its Density which those
very dense Substances have in respect of theirs, excepting so far as
those differ from one another.

Again, the Refraction of Camphire, Oil Olive, Linseed Oil, Spirit of
Turpentine and Amber, which are fat sulphureous unctuous Bodies, and a
Diamond, which probably is an unctuous Substance coagulated, have their
refractive Po