into the Prism at the Plane Side FIGK, and reflected in it by its Base
HEIG, and thence going out through its Plane Side HEFK to the Eye. And
when the Prism and Eye are conveniently placed, so that the Angles of
Incidence and Reflexion at the Base may be about 40 Degrees, the
Spectator will see a Bow MN of a blue Colour, running from one End of
the Base to the other, with the Concave Side towards him, and the Part
of the Base IMNG beyond this Bow will be brighter than the other Part
EMNH on the other Side of it. This blue Colour MN being made by nothing
else than by Reflexion of a specular Superficies, seems so odd a
Ph nomenon, and so difficult to be explained by the vulgar Hypothesis of
Philosophers, that I could not but think it deserved to be taken Notice
of. Now for understanding the Reason of it, suppose the Plane ABC to cut
the Plane Sides and Base of the Prism perpendicularly. From the Eye to
the Line BC, wherein that Plane cuts the Base, draw the Lines S_p_ and
S_t_, in the Angles S_pc_ 50 degr. 1/9, and S_tc_ 49 degr. 1/28, and the
Point _p_ will be the Limit beyond which none of the most refrangible
Rays can pass through the Base of the Prism, and be refracted, whose
Incidence is such that they may be reflected to the Eye; and the Point
_t_ will be the like Limit for the least refrangible Rays, that is,
beyond which none of them can pass through the Base, whose Incidence is
such that by Reflexion they may come to the Eye. And the Point _r_ taken
in the middle Way between _p_ and _t_, will be the like Limit for the
meanly refrangible Rays. And therefore all the least refrangible Rays
which fall upon the Base beyond _t_, that is, between _t_ and B, and can
come from thence to the Eye, will be reflected thither: But on this side
_t_, that is, between _t_ and _c_, many of these Rays will be
transmitted through the Base. And all the most refrangible Rays which
fall upon the Base beyond _p_, that is, between, _p_ and B, and can by
Reflexion come from thence to the Eye, will be reflected thither, but
every where between _p_ and _c_, many of these Rays will get through the
Base, and be refracted; and the same is to be understood of the meanly
refrangible Rays on either side of the Point _r_. Whence it follows,
that the Base of the Prism must every where between _t_ and B, by a
total Reflexion of all sorts of Rays to the Eye, look white and bright.
And every where between _p_ and C, by reason of the Transmission of many
Rays of every sort, look more pale, obscure, and dark. But at _r_, and
in other Places between _p_ and _t_, where all the more refrangible Rays
are reflected to the Eye, and many of the less refrangible are
transmitted, the Excess of the most refrangible in the reflected Light
will tinge that Light with their Colour, which is violet and blue. And
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