above; he confessed himself at a loss for explaining them. For Pressions
or Motions, propagated from a shining Body through an uniform Medium,
must be on all sides alike; whereas by those Experiments it appears,
that the Rays of Light have different Properties in their different
Sides. He suspected that the Pulses of _ ther_ in passing through the
first Crystal might receive certain new Modifications, which might
determine them to be propagated in this or that Medium within the
second Crystal, according to the Position of that Crystal. But what
Modifications those might be he could not say, nor think of any thing
satisfactory in that Point. And if he had known that the unusual
Refraction depends not on new Modifications, but on the original and
unchangeable Dispositions of the Rays, he would have found it as
difficult to explain how those Dispositions which he supposed to be
impress'd on the Rays by the first Crystal, could be in them before
their Incidence on that Crystal, and in general, how all Rays emitted by
shining Bodies, can have those Dispositions in them from the beginning.
To me, at least, this seems inexplicable, if Light be nothing else than
Pression or Motion propagated through _ ther_.
And it is as difficult to explain by these Hypotheses, how Rays can be
alternately in Fits of easy Reflexion and easy Transmission; unless
perhaps one might suppose that there are in all Space two thereal
vibrating Mediums, and that the Vibrations of one of them constitute
Light, and the Vibrations of the other are swifter, and as often as they
overtake the Vibrations of the first, put them into those Fits. But how
two _ thers_ can be diffused through all Space, one of which acts upon
the other, and by consequence is re-acted upon, without retarding,
shattering, dispersing and confounding one anothers Motions, is
inconceivable. And against filling the Heavens with fluid Mediums,
unless they be exceeding rare, a great Objection arises from the regular
and very lasting Motions of the Planets and Comets in all manner of
Courses through the Heavens. For thence it is manifest, that the Heavens
are void of all sensible Resistance, and by consequence of all sensible
Matter.
For the resisting Power of fluid Mediums arises partly from the
Attrition of the Parts of the Medium, and partly from the _Vis inerti _
of the Matter. That part of the Resistance of a spherical Body which
arises from the Attrition of the Parts of the Medium is very nearly as
the Diameter, or, at the most, as the _Factum_ of the Diameter, and the
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