contrary way, that the least refrangible Rays may be now more copiously
reflected than the rest, and by their Excess the Whiteness will be
changed into yellow and red. The several Rays therefore in that white
Light do retain their colorific Qualities, by which those of any sort,
whenever they become more copious than the rest, do by their Excess and
Predominance cause their proper Colour to appear.
And by the same way of arguing, applied to the third Experiment of this
second Part of the first Book, it may be concluded, that the white
Colour of all refracted Light at its very first Emergence, where it
appears as white as before its Incidence, is compounded of various
Colours.
[Illustration: FIG. 10.]
_Exper._ 13. In the foregoing Experiment the several Intervals of the
Teeth of the Comb do the Office of so many Prisms, every Interval
producing the Ph nomenon of one Prism. Whence instead of those Intervals
using several Prisms, I try'd to compound Whiteness by mixing their
Colours, and did it by using only three Prisms, as also by using only
two as follows. Let two Prisms ABC and _abc_, [in _Fig._ 10.] whose
refracting Angles B and _b_ are equal, be so placed parallel to one
another, that the refracting Angle B of the one may touch the Angle _c_
at the Base of the other, and their Planes CB and _cb_, at which the
Rays emerge, may lie in Directum. Then let the Light trajected through
them fall upon the Paper MN, distant about 8 or 12 Inches from the
Prisms. And the Colours generated by the interior Limits B and _c_ of
the two Prisms, will be mingled at PT, and there compound white. For if
either Prism be taken away, the Colours made by the other will appear in
that Place PT, and when the Prism is restored to its Place again, so
that its Colours may there fall upon the Colours of the other, the
Mixture of them both will restore the Whiteness.
This Experiment succeeds also, as I have tried, when the Angle _b_ of
the lower Prism, is a little greater than the Angle B of the upper, and
between the interior Angles B and _c_, there intercedes some Space B_c_,
as is represented in the Figure, and the refracting Planes BC and _bc_,
are neither in Directum, nor parallel to one another. For there is
nothing more requisite to the Success of this Experiment, than that the
Rays of all sorts may be uniformly mixed upon the Paper in the Place PT.
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