Alteration in its Modifications and Properties. I do not here take
Notice of any Refractions made in the sides of the first Prism, because
the Light enters it perpendicularly at the first side, and goes out
perpendicularly at the second side, and therefore suffers none. So then,
the Sun's incident Light being of the same Temper and Constitution with
his emergent Light, and the last being compounded of Rays differently
refrangible, the first must be in like manner compounded.
[Illustration: FIG. 21.]
_Illustration._ In the twenty-first Figure, ABC is the first Prism, BC
its Base, B and C its equal Angles at the Base, each of 45 Degrees, A
its rectangular Vertex, FM a beam of the Sun's Light let into a dark
Room through a hole F one third part of an Inch broad, M its Incidence
on the Base of the Prism, MG a less refracted Ray, MH a more refracted
Ray, MN the beam of Light reflected from the Base, VXY the second Prism
by which this beam in passing through it is refracted, N_t_ the less
refracted Light of this beam, and N_p_ the more refracted part thereof.
When the first Prism ABC is turned about its Axis according to the order
of the Letters ABC, the Rays MH emerge more and more obliquely out of
that Prism, and at length after their most oblique Emergence are
reflected towards N, and going on to _p_ do increase the Number of the
Rays N_p_. Afterwards by continuing the Motion of the first Prism, the
Rays MG are also reflected to N and increase the number of the Rays
N_t_. And therefore the Light MN admits into its Composition, first the
more refrangible Rays, and then the less refrangible Rays, and yet after
this Composition is of the same Nature with the Sun's immediate Light
FM, the Reflexion of the specular Base BC causing no Alteration therein.
_Exper._ 10. Two Prisms, which were alike in Shape, I tied so together,
that their Axis and opposite Sides being parallel, they composed a
Parallelopiped. And, the Sun shining into my dark Chamber through a
little hole in the Window-shut, I placed that Parallelopiped in his beam
at some distance from the hole, in such a Posture, that 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
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