whose two Sides through which the Light passed to the Eye were plane and
well polished, and contained an Angle of about sixty degrees; which
Angle I call the refracting Angle of the Prism. And whilst I view'd it,
I held it and the Prism before a Window in such manner that the Sides of
the Paper were parallel to the Prism, and both those Sides and the Prism
were parallel to the Horizon, and the cross Line was also parallel to
it: and that the Light which fell from the Window upon the Paper made an
Angle with the Paper, equal to that Angle which was made with the same
Paper by the Light reflected from it to the Eye. Beyond the Prism was
the Wall of the Chamber under the Window covered over with black Cloth,
and the Cloth was involved in Darkness that no Light might be reflected
from thence, which in passing by the Edges of the Paper to the Eye,
might mingle itself with the Light of the Paper, and obscure the
Ph nomenon thereof. These things being thus ordered, I found that if the
refracting Angle of the Prism be turned upwards, so that the Paper may
seem to be lifted upwards by the Refraction, its blue half will be
lifted higher by the Refraction than its red half. But if the refracting
Angle of the Prism be turned downward, so that the Paper may seem to be
carried lower by the Refraction, its blue half will be carried something
lower thereby than its red half. Wherefore in both Cases the Light which
comes from the blue half of the Paper through the Prism to the Eye, does
in like Circumstances suffer a greater Refraction than the Light which
comes from the red half, and by consequence is more refrangible.
_Illustration._ In the eleventh Figure, MN represents the Window, and DE
the Paper terminated with parallel Sides DJ and HE, and by the
transverse Line FG distinguished into two halfs, the one DG of an
intensely blue Colour, the other FE of an intensely red. And BAC_cab_
represents the Prism whose refracting Planes AB_ba_ and AC_ca_ meet in
the Edge of the refracting Angle A_a_. This Edge A_a_ being upward, is
parallel both to the Horizon, and to the Parallel-Edges of the Paper DJ
and HE, and the transverse Line FG is perpendicular to the Plane of the
Window. And _de_ represents the Image of the Paper seen by Refraction
upwards in such manner, that the blue half DG is carried higher to _dg_
than the red half FE is to _fe_, and therefore suffers a greater
Refraction. If the Edge of the refracting Angle be turned downward, the
Image of the Paper will be refracted downward; suppose to [Greek: de],
and the blue half will be refracted lower to [Greek: dg] than the red
half is to [Greek: pe].
15