explain'd in the first, fourth, and eighth Observations of the first
Part of the second Book. But, if the second Glass be taken away, the
Light which goes out of the second Surface of the first Glass into the
Air or _Vacuum_, will not go on forwards, but turns back into the first
Glass, and is reflected; and therefore it is drawn back by the Power of
the first Glass, there being nothing else to turn it back. Nothing more
is requisite for producing all the variety of Colours, and degrees of
Refrangibility, than that the Rays of Light be Bodies of different
Sizes, the least of which may take violet the weakest and darkest of the
Colours, and be more easily diverted by refracting Surfaces from the
right Course; and the rest as they are bigger and bigger, may make the
stronger and more lucid Colours, blue, green, yellow, and red, and be
more and more difficultly diverted. Nothing more is requisite for
putting the Rays of Light into Fits of easy Reflexion and easy
Transmission, than that they be small Bodies which by their attractive
Powers, or some other Force, stir up Vibrations in what they act upon,
which Vibrations being swifter than the Rays, overtake them
successively, and agitate them so as by turns to increase and decrease
their Velocities, and thereby put them into those Fits. And lastly, the
unusual Refraction of Island-Crystal looks very much as if it were
perform'd by some kind of attractive virtue lodged in certain Sides both
of the Rays, and of the Particles of the Crystal. For were it not for
some kind of Disposition or Virtue lodged in some Sides of the Particles
of the Crystal, and not in their other Sides, and which inclines and
bends the Rays towards the Coast of unusual Refraction, the Rays which
fall perpendicularly on the Crystal, would not be refracted towards that
Coast rather than towards any other Coast, both at their Incidence and
at their Emergence, so as to emerge perpendicularly by a contrary
Situation of the Coast of unusual Refraction at the second Surface; the
Crystal acting upon the Rays after they have pass'd through it, and are
emerging into the Air; or, if you please, into a _Vacuum_. And since the
Crystal by this Disposition or Virtue does not act upon the Rays, unless
when one of their Sides of unusual Refraction looks towards that Coast,
this argues a Virtue or Disposition in those Sides of the Rays, which
answers to, and sympathizes with that Virtue or Disposition of the
Crystal, as the Poles of two Magnets answer to one another. And as
Magnetism may be intended and remitted, and is found only in the Magnet
and in Iron: So this Virtue of refracting the perpendicular Rays is
greater in Island-Crystal, less in Crystal of the Rock, and is not yet
found in other Bodies. I do not say that this Virtue is magnetical: It
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