this happens by taking the Line C _prt_ B any where between the Ends of
the Prism HG and EI.
_PROP._ IX. PROB. IV.
_By the discovered Properties of Light to explain the Colours of the
Rain-bow._
[Illustration: FIG. 14.]
This Bow never appears, but where it rains in the Sun-shine, and may be
made artificially by spouting up Water which may break aloft, and
scatter into Drops, and fall down like Rain. For the Sun shining upon
these Drops certainly causes the Bow to appear to a Spectator standing
in a due Position to the Rain and Sun. And hence it is now agreed upon,
that this Bow is made by Refraction of the Sun's Light in drops of
falling Rain. This was understood by some of the Antients, and of late
more fully discover'd and explain'd by the famous _Antonius de Dominis_
Archbishop of _Spalato_, in his book _De Radiis Visus & Lucis_,
published by his Friend _Bartolus_ at _Venice_, in the Year 1611, and
written above 20 Years before. For he teaches there how the interior Bow
is made in round Drops of Rain by two Refractions of the Sun's Light,
and one Reflexion between them, and the exterior by two Refractions, and
two sorts of Reflexions between them in each Drop of Water, and proves
his Explications by Experiments made with a Phial full of Water, and
with Globes of Glass filled with Water, and placed in the Sun to make
the Colours of the two Bows appear in them. The same Explication
_Des-Cartes_ hath pursued in his Meteors, and mended that of the
exterior Bow. But whilst they understood not the true Origin of Colours,
it's necessary to pursue it here a little farther. For understanding
therefore how the Bow is made, let a Drop of Rain, or any other
spherical transparent Body be represented by the Sphere BNFG, [in _Fig._
14.] described with the Center C, and Semi-diameter CN. And let AN be
one of the Sun's Rays incident upon it at N, and thence refracted to F,
where let it either go out of the Sphere by Refraction towards V, or be
reflected to G; and at G let it either go out by Refraction to R, or be
reflected to H; and at H let it go out by Refraction towards S, cutting
the incident Ray in Y. Produce AN and RG, till they meet in X, and upon
AX and NF, let fall the Perpendiculars CD and CE, and produce CD till it
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