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.

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