here the thickness of the Glass determines whether that Power by which
Glass acts upon Light shall cause it to be reflected, or suffer it to
be transmitted. And, Thirdly, because those Surfaces of transparent
Bodies which have the greatest refracting power, reflect the greatest
quantity of Light, as was shewn in the first Proposition.
PROP. X.
_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
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