iment mention'd in the 25th Question, was only in the Positions of
the Sides of the Rays to the Planes of perpendicular Refraction. For one
and the same Ray is here refracted sometimes after the usual, and
sometimes after the unusual manner, according to the Position which its
Sides have to the Crystals. If the Sides of the Ray are posited the same
way to both Crystals, it is refracted after the same manner in them
both: But if that side of the Ray which looks towards the Coast of the
unusual Refraction of the first Crystal, be 90 Degrees from that side of
the same Ray which looks toward the Coast of the unusual Refraction of
the second Crystal, (which may be effected by varying the Position of
the second Crystal to the first, and by consequence to the Rays of
Light,) the Ray shall be refracted after several manners in the several
Crystals. There is nothing more required to determine whether the Rays
of Light which fall upon the second Crystal shall be refracted after
the usual or after the unusual manner, but to turn about this Crystal,
so that the Coast of this Crystal's unusual Refraction may be on this or
on that side of the Ray. And therefore every Ray may be consider'd as
having four Sides or Quarters, two of which opposite to one another
incline the Ray to be refracted after the unusual manner, as often as
either of them are turn'd towards the Coast of unusual Refraction; and
the other two, whenever either of them are turn'd towards the Coast of
unusual Refraction, do not incline it to be otherwise refracted than
after the usual manner. The two first may therefore be call'd the Sides
of unusual Refraction. And since these Dispositions were in the Rays
before their Incidence on the second, third, and fourth Surfaces of the
two Crystals, and suffered no alteration (so far as appears,) by the
Refraction of the Rays in their passage through those Surfaces, and the
Rays were refracted by the same Laws in all the four Surfaces; it
appears that those Dispositions were in the Rays originally, and
suffer'd no alteration by the first Refraction, and that by means of
those Dispositions the Rays were refracted at their Incidence on the
first Surface of the first Crystal, some of them after the usual, and
some of them after the unusual manner, accordingly as their Sides of
unusual Refraction were then turn'd towards the Coast of the unusual
Refraction of that Crystal, or sideways from it.

Every Ray of Light has therefore two