_If Light be swifter in Bodies than in Vacuo, in the proportion of the
Sines which measure the Refraction of the Bodies, the Forces of the
Bodies to reflect and refract Light, are very nearly proportional to the
densities of the same Bodies; excepting that unctuous and sulphureous
Bodies refract more than others of this same density._
[Illustration: FIG. 8.]
Let AB represent the refracting plane Surface of any Body, and IC a Ray
incident very obliquely upon the Body in C, so that the Angle ACI may be
infinitely little, and let CR be the refracted Ray. From a given Point B
perpendicular to the refracting Surface erect BR meeting with the
refracting Ray CR in R, and if CR represent the Motion of the refracted
Ray, and this Motion be distinguish'd into two Motions CB and BR,
whereof CB is parallel to the refracting Plane, and BR perpendicular to
it: CB shall represent the Motion of the incident Ray, and BR the
Motion generated by the Refraction, as Opticians have of late explain'd.
Now if any Body or Thing, in moving through any Space of a given breadth
terminated on both sides by two parallel Planes, be urged forward in all
parts of that Space by Forces tending directly forwards towards the last
Plane, and before its Incidence on the first Plane, had no Motion
towards it, or but an infinitely little one; and if the Forces in all
parts of that Space, between the Planes, be at equal distances from the
Planes equal to one another, but at several distances be bigger or less
in any given Proportion, the Motion generated by the Forces in the whole
passage of the Body or thing through that Space shall be in a
subduplicate Proportion of the Forces, as Mathematicians will easily
understand. And therefore, if the Space of activity of the refracting
Superficies of the Body be consider'd as such a Space, the Motion of the
Ray generated by the refracting Force of the Body, during its passage
through that Space, that is, the Motion BR, must be in subduplicate
Proportion of that refracting Force. I say therefore, that the Square of
the Line BR, and by consequence the refracting Force of the Body, is
very nearly as the density of the same Body. For this will appear by the
following Table, wherein the Proportion of the Sines which measure the
Refractions of several Bodies, the Square of BR, supposing CB an unite,
the Densities of the Bodies estimated by their Specifick Gravities, and
their Refractive Power in respect of their Densities are set down in
several Columns.
---------------------+----------------+----------------+----------+-----------
| | | |
| | The Square | The | The
| | of BR, to | density | refractive
| The Proportion | which the | and | Power of
| of the Sines of| refracting | specifick| the Body
| Incidence and | force of the | gravity | in respect
The refracting | Refraction of | Body is | of the | of its
Bodies. | yellow Light. | proportionate. | Body. | density.
---------------------+----------------+----------------+----------+-----------
A Pseudo-Topazius, | | | |
being a natural, | | | |
pellucid, brittle, | 23 to 14 | 1'699 | 4'27 | 3979
hairy Stone, of a | | | |
yellow Colour. | | | |
Air. | 3201 to 3200 | 0'000625 | 0'0012 | 5208
Glass of Antimony. | 17 to 9 | 2'568 | 5'28 | 4864
A Selenitis. | 61 to 41 | 1'213 | 2'252 | 5386
Glass vulgar. | 31 to 20 | 1'4025 | 2'58 | 5436
Crystal of the Rock. | 25 to 16 | 1'445 | 2'65 | 5450
Island Crystal. | 5 to 3 | 1'778 | 2'72 | 6536
Sal Gemmae. | 17 to 11 | 1'388 | 2'143 | 6477
Alume. | 35 to 24 | 1'1267 | 1'714 | 6570
Borax. | 22 to 15 | 1'1511 | 1'714 | 6716
Niter. | 32 to 21 | 1'345 | 1'9 | 7079
Dantzick Vitriol. | 303 to 200 | 1'295 | 1'715 | 7551
Oil of Vitriol. | 10 to 7 | 1'041 | 1'7 | 6124
Rain Water. | 529 to 396 | 0'7845 | 1' | 7845
Gum Arabick. | 31 to 21 | 1'179 | 1'375 | 8574
Spirit of Wine well | | | |
rectified. | 100 to 73 | 0'8765 | 0'866 | 10121
Camphire. | 3 to 2 | 1'25 | 0'996 | 12551
Oil Olive. | 22 to 15 | 1'1511 | 0'913 | 12607
Linseed Oil. | 40 to 27 | 1'1948 | 0'932 | 12819
Spirit of Turpentine.| 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 Powers in Proportion to one another as their Densities
without any considerable Variation. But the refractive Powers of these
unctuous Substances are two or three Times greater in respect of their
Densities than the refractive Powers of the former Substances in respect
of theirs.
Water has a refractive Power in a middle degree between those two sorts
of Substances, and probably is of a middle nature. For out of it grow
all vegetable and animal Substances, which consist as well of
sulphureous fat and inflamable Parts, as of earthy lean and alcalizate
ones.
Salts and Vitriols have refractive Powers in a middle degree between
those of earthy Substances and Water, and accordingly are composed of
those two sorts of Substances. For by distillation and rectification of
their Spirits a great Part of them goes into Water, and a great Part
remains behind in the form of a dry fix'd Earth capable of
Vitrification.
Spirit of Wine has a refractive Power in a middle degree between those
of Water and oily Substances, and accordingly seems to be composed of
both, united by Fermentation; the Water, by means of some saline Spirits
with which 'tis impregnated, dissolving the Oil, and volatizing it by
the Action. For Spirit of Wine is inflamable by means of its oily Parts,
and being distilled often from Salt of Tartar, grow by every
distillation more and more aqueous and phlegmatick. And Chymists
observe, that Vegetables (as Lavender, Rue, Marjoram, &c.) distilled
_per se_, before fermentation yield Oils without any burning Spirits,
but after fermentation yield ardent Spirits without Oils: Which shews,
that their Oil is by fermentation converted into Spirit. They find also,
that if Oils be poured in a small quantity upon fermentating Vegetables,
they distil over after fermentation in the form of Spirits.
So then, by the foregoing Table, all Bodies seem to have their
refractive Powers proportional to their Densities, (or very nearly;)
excepting so far as they partake more or less of sulphureous oily
Particles, and thereby have their refractive Power made greater or less.
Whence it seems rational to attribute the refractive Power of all Bodies
chiefly, if not wholly, to the sulphureous Parts with which they abound.
For it's probable that all Bodies abound more or less with Sulphurs. And
as Light congregated by a Burning-glass acts most upon sulphureous
Bodies, to turn them into Fire and Flame; so, since all Action is
mutual, Sulphurs ought to act most upon Light. For that the action
between Light and Bodies is mutual, may appear from this Consideration;
That the densest Bodies which refract and reflect Light most strongly,
grow hottest in the Summer Sun, by the action of the refracted or
reflected Light.
I have hitherto explain'd the power of Bodies to reflect and refract,
and shew'd, that thin transparent Plates, Fibres, and Particles, do,
according to their several thicknesses and densities, reflect several
sorts of Rays, and thereby appear of several Colours; and by consequence
that nothing more is requisite for producing all the Colours of natural
Bodies, than the several sizes and densities of their transparent
Particles. But whence it is that these Plates, Fibres, and Particles,
do, according to their several thicknesses and densities, reflect
several sorts of Rays, I have not yet explain'd. To give some insight
into this matter, and make way for understanding the next part of this
Book, I shall conclude this part with a few more Propositions. Those
which preceded respect the nature of Bodies, these the nature of Light:
For both must be understood, before the reason of their Actions upon one
another can be known. And because the last Proposition depended upon the
velocity of Light, I will begin with a Proposition of that kind.