Next, if they be
_Globular_, and mov'd only with a _turbinated_ motion, I know not any cause
that can impress that motion upon the _pellucid medium_, which yet is done.
Thirdly, any other _irregular_ motion of the parts one amongst another,
must necessarily make the body of a fluid consistence, from which it is far
enough. It must therefore be a _Vibrating_ motion.
And Thirdly, That it is a very _short-vibrating motion_, I think the
instances drawn from the shining of Diamonds will also make probable. For a
Diamond being the hardest body we yet know in the World, and consequently
the least apt to yield or bend, must consequently also have its
_vibrations_ exceeding short.
And these, I think, are the three principal proprieties of a motion,
requisite to produce the effect call'd Light in the Object.
The next thing we are to consider, is the way or manner of the _trajection_
of this motion through the interpos'd pellucid body to the eye: And here it
will be easily granted,
First, That it must be a body _susceptible_ and _impartible_ of this motion
that will deserve the name of a Transparent. And next, that the parts of
such a body must be _Homogeneous_, or of the same kind. Thirdly, that the
constitution and motion of the parts must be such, that the appulse of the
luminous body may be communicated or propagated through it to the greatest
imaginable distance in the least imaginable time, though I see no reason to
affirm, that it must be in an instant: For I know not any one Experiment or
observation that does prove it.
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