and reflects them. And this is still more evident by
laying together two Prisms of Glass, or two Object-glasses of very long
Telescopes, the one plane, the other a little convex, and so compressing
them that they do not fully touch, nor are too far asunder. For the
Light which falls upon the farther Surface of the first Glass where the
Interval between the Glasses is not above the ten hundred thousandth
Part of an Inch, will go through that Surface, and through the Air or
_Vacuum_ between the Glasses, and enter into the second Glass, as was
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 em