the Axes of the
Prisms might be perpendicular to the incident Rays, and that those Rays
being incident upon the first Side of one Prism, might go on through the
two contiguous Sides of both Prisms, and emerge out of the last Side of
the second Prism. This Side being parallel to the first Side of the
first Prism, caused the emerging Light to be parallel to the incident.
Then, beyond these two Prisms I placed a third, which might refract that
emergent Light, and by that Refraction cast the usual Colours of the
Prism upon the opposite Wall, or upon a sheet of white Paper held at a
convenient Distance behind the Prism for that refracted Light to fall
upon it. After this I turned the Parallelopiped about its Axis, and
found that when the contiguous Sides of the two Prisms became so oblique
to the incident Rays, that those Rays began all of them to be
reflected, those Rays which in the third Prism had suffered the greatest
Refraction, and painted the Paper with violet and blue, were first of
all by a total Reflexion taken out of the transmitted Light, the rest
remaining and on the Paper painting their Colours of green, yellow,
orange and red, as before; and afterwards by continuing the Motion of
the two Prisms, the rest of the Rays also by a total Reflexion vanished
in order, according to their degrees of Refrangibility. The Light
therefore which emerged out of the two Prisms is compounded of Rays
differently refrangible, seeing the more refrangible Rays may be taken
out of it, while the less refrangible remain. But this Light being
trajected only through the parallel Superficies of the two Prisms, if it
suffer'd any change by the Refraction of one Superficies it lost that
Impression by the contrary Refraction of the other Superficies, and so
being restor'd to its pristine Constitution, became of the same Nature
and Condition as at first before its Incidence on those Prisms; and
therefore, before its Incidence, was as much compounded of Rays
differently refrangible, as afterwards.

[Illustration: FIG. 22.]

_Illustration._ In the twenty second Figure ABC and BCD are the two
Prisms tied together in the form of a Parallelopiped, their Sides BC and
CB being contiguous, and their Sides AB and CD parallel. And HJK is the
third Prism, by which the Sun's Light propagated through the hole F into
the dark Chamber, and there passing through those sides of the Prisms
AB, BC, CB and CD, is refracted at O to the white Paper PT, falling