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.
PROP. XI.
_Light is propagated from luminous Bodies in time, and spends about
seven or eight Minutes of an Hour in passing from the Sun to the Earth._
This was observed first by _Roemer_, and then by others, by means of the
Eclipses of the Satellites of _Jupiter_. For these Eclipses, when the
Earth is between the Sun and _Jupiter_, happen about seven or eight
Minutes sooner than they ought to do by the Tables, and when the Earth
is beyond the Sun they happen about seven or eight Minutes later than
they ought to do; the reason being, that the Light of the Satellites has
farther to go in the latter case than in the former by the Diameter of
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