So, for instance, having with a Lens 4-1/4 Inches broad, and two Prisms
on either hand 6-1/4 Feet distant from the Lens, made such a beam of
compounded Light; to examine the reason of the Colours made by Prisms, I
refracted this compounded beam of Light XY with another Prism HIK _kh_,
and thereby cast the usual Prismatick Colours PQRST upon the Paper LV
placed behind. And then by stopping any of the Colours _p_, _q_, _r_,
_s_, _t_, at the Lens, I found that the same Colour would vanish at the
Paper. So if the Purple _p_ was stopp'd at the Lens, the Purple P upon
the Paper would vanish, and the rest of the Colours would remain
unalter'd, unless perhaps the blue, so far as some purple latent in it
at the Lens might be separated from it by the following Refractions. And
so by intercepting the green upon the Lens, the green R upon the Paper
would vanish, and so of the rest; which plainly shews, that as the white
beam of Light XY was compounded of several Lights variously colour'd at
the Lens, so the Colours which afterwards emerge out of it by new
Refractions are no other than those of which its Whiteness was
compounded. The Refraction of the Prism HIK _kh_ generates the Colours
PQRST upon the Paper, not by changing the colorific Qualities of the
Rays, but by separating the Rays which had the very same colorific
Qualities before they enter'd the Composition of the refracted beam of
white Light XY. For otherwise the Rays which were of one Colour at the
Lens might be of another upon the Paper, contrary to what we find.
So again, to examine the reason of the Colours of natural Bodies, I
placed such Bodies in the Beam of Light XY, and found that they all
appeared there of those their own Colours which they have in Day-light,
and that those Colours depend upon the Rays which had the same Colours
at the Lens before they enter'd the Composition of that beam. Thus, for
instance, Cinnaber illuminated by this beam appears of the same red
Colour as in Day-light; and if at the Lens you intercept the
green-making and blue-making Rays, its redness will become more full and
lively: But if you there intercept the red-making Rays, it will not any
longer appear red, but become yellow or green, or of some other Colour,
according to the sorts of Rays which you do not intercept. So Gold in
this Light XY appears of the same yellow Colour as in Day-light, but by
intercepting at the Lens a due Quantity of the yellow-making Rays it
will appear white like Silver (as I have tried) which shews that its
yellowness arises from the Excess of the intercepted Rays tinging that
Whiteness with their Colour when they are let pass. So the Infusion of
_Lignum Nephriticum_ (as I have also tried) when held in this beam of
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