opposite Sides, originally endued
with a Property on which the unusual Refraction depends, and the other
two opposite Sides not endued with that Property. And it remains to be
enquired, whether there are not more Properties of Light by which the
Sides of the Rays differ, and are distinguished from one another.

In explaining the difference of the Sides of the Rays above mention'd, I
have supposed that the Rays fall perpendicularly on the first Crystal.
But if they fall obliquely on it, the Success is the same. Those Rays
which are refracted after the usual manner in the first Crystal, will be
refracted after the unusual manner in the second Crystal, supposing the
Planes of perpendicular Refraction to be at right Angles with one
another, as above; and on the contrary.

If the Planes of the perpendicular Refraction of the two Crystals be
neither parallel nor perpendicular to one another, but contain an acute
Angle: The two beams of Light which emerge out of the first Crystal,
will be each of them divided into two more at their Incidence on the
second Crystal. For in this case the Rays in each of the two beams will
some of them have their Sides of unusual Refraction, and some of them
their other Sides turn'd towards the Coast of the unusual Refraction of
the second Crystal.

_Qu._ 27. Are not all Hypotheses erroneous which have hitherto been
invented for explaining the Phaenomena of Light, by new Modifications of
the Rays? For those Phaenomena depend not upon new Modifications, as has
been supposed, but upon the original and unchangeable Properties of the
Rays.

_Qu._ 28. Are not all Hypotheses erroneous, in which Light is supposed
to consist in Pression or Motion, propagated through a fluid Medium? For
in all these Hypotheses the Phaenomena of Light have been hitherto
explain'd by supposing that they arise from new Modifications of the
Rays; which is an erroneous Supposition.

If Light consisted only in Pression propagated without actual Motion, it
would not be able to agitate and heat the Bodies which refract and
reflect it. If it consisted in Motion propagated to all distances in an
instant, it would require an infinite force every moment, in every
shining Particle, to generate that Motion. And if it consisted in
Pression or Motion, propagated either in an instant or in time, it would
bend into the Shadow. For Pression or Motion cannot be propagated in a
Fluid in right Lines, beyond an Obstacle which stops part of