Line with their ends contiguous as before, but be broken off from one
another and become parallel, the blue end _m_ of the Image _mn_ being by
a greater Refraction translated farther from its former place MT, than
the red end _t_ of the other Image _pt_ from the same place MT; which
puts the Proposition past Dispute. And this happens whether the third
Prism DH be placed immediately after the two first, or at a great
distance from them, so that the Light refracted in the two first Prisms
be either white and circular, or coloured and oblong when it falls on
the third.
_Exper._ 6. In the middle of two thin Boards I made round holes a third
part of an Inch in diameter, and in the Window-shut a much broader hole
being made to let into my darkned Chamber a large Beam of the Sun's
Light; I placed a Prism behind the Shut in that beam to refract it
towards the opposite Wall, and close behind the Prism I fixed one of the
Boards, in such manner that the middle of the refracted Light might pass
through the hole made in it, and the rest be intercepted by the Board.
Then at the distance of about twelve Feet from the first Board I fixed
the other Board in such manner that the middle of the refracted Light
which came through the hole in the first Board, and fell upon the
opposite Wall, might pass through the hole in this other Board, and the
rest being intercepted by the Board might paint upon it the coloured
Spectrum of the Sun. And close behind this Board I fixed another Prism
to refract the Light which came through the hole. Then I returned
speedily to the first Prism, and by turning it slowly to and fro about
its Axis, I caused the Image which fell upon the second Board to move up
and down upon that Board, that all its parts might successively pass
through the hole in that Board and fall upon the Prism behind it. And in
the mean time, I noted the places on the opposite Wall to which that
Light after its Refraction in the second Prism did pass; and by the
difference of the places I found that the Light which being most
refracted in the first Prism did go to the blue end of the Image, was
again more refracted in the second Prism than the Light which went to
the red end of that Image, which proves as well the first Proposition as
the second. And this happened whether the Axis of the two Prisms were
parallel, or inclined to one another, and to the Horizon in any given
Angles.
_Illustration._ Let F [in _Fig._ 18.] be the wide hole in the
Window-shut, through which the Sun shines upon the first Prism ABC, and
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