the second. And seeing all these things continue to succeed after the
same manner when the Rays are again in a third Prism, and again in a
fourth refracted sideways, it is evident that the Rays of one and the
same Circle, as to their degree of Refrangibility, continue always
uniform and homogeneal to one another, and that those of several Circles
do differ in degree of Refrangibility, and that in some certain and
constant Proportion. Which is the thing I was to prove.
There is yet another Circumstance or two of this Experiment by which it
becomes still more plain and convincing. Let the second Prism DH [in
_Fig._ 16.] be placed not immediately after the first, but at some
distance from it; suppose in the mid-way between it and the Wall on
which the oblong Spectrum PT is cast, so that the Light from the first
Prism may fall upon it in the form of an oblong Spectrum [Greek: pt]
parallel to this second Prism, and be refracted sideways to form the
oblong Spectrum _pt_ upon the Wall. And you will find as before, that
this Spectrum _pt_ is inclined to that Spectrum PT, which the first
Prism forms alone without the second; the blue ends P and _p_ being
farther distant from one another than the red ones T and _t_, and by
consequence that the Rays which go to the blue end [Greek: p] of the
Image [Greek: pt], and which therefore suffer the greatest Refraction in
the first Prism, are again in the second Prism more refracted than the
rest.
[Illustration: FIG. 16.]
[Illustration: FIG. 17.]
The same thing I try'd also by letting the Sun's Light into a dark Room
through two little round holes F and [Greek: ph] [in _Fig._ 17.] made in
the Window, and with two parallel Prisms ABC and [Greek: abg] placed at
those holes (one at each) refracting those two beams of Light to the
opposite Wall of the Chamber, in such manner that the two colour'd
Images PT and MN which they there painted were joined end to end and lay
in one straight Line, the red end T of the one touching the blue end M
of the other. For if these two refracted Beams were again by a third
Prism DH placed cross to the two first, refracted sideways, and the
Spectrums thereby translated to some other part of the Wall of the
Chamber, suppose the Spectrum PT to _pt_ and the Spectrum MN to _mn_,
these translated Spectrums _pt_ and _mn_ would not lie in one straight
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