several sorts of Rays are mixed, and in crossing pass through the same
space, they do not act on one another so as to change each others
colorific qualities. (_Exper._ 10. _Part_ 2.) but by mixing their
Actions in the Sensorium beget a Sensation differing from what either
would do apart, that is a Sensation of a mean Colour between their
proper Colours; and particularly when by the concourse and mixtures of
all sorts of Rays, a white Colour is produced, the white is a mixture of
all the Colours which the Rays would have apart, (_Prop._ 5. _Part_ 2.)
The Rays in that mixture do not lose or alter their several colorific
qualities, but by all their various kinds of Actions mix'd in the
Sensorium, beget a Sensation of a middling Colour between all their
Colours, which is whiteness. For whiteness is a mean between all
Colours, having it self indifferently to them all, so as with equal
facility to be tinged with any of them. A red Powder mixed with a little
blue, or a blue with a little red, doth not presently lose its Colour,
but a white Powder mix'd with any Colour is presently tinged with that
Colour, and is equally capable of being tinged with any Colour whatever.
It has been shewed also, that as the Sun's Light is mix'd of all sorts
of Rays, so its whiteness is a mixture of the Colours of all sorts of
Rays; those Rays having from the beginning their several colorific
qualities as well as their several Refrangibilities, and retaining them
perpetually unchanged notwithstanding any Refractions or Reflexions they
may at any time suffer, and that whenever any sort of the Sun's Rays is
by any means (as by Reflexion in _Exper._ 9, and 10. _Part_ 1. or by
Refraction as happens in all Refractions) separated from the rest, they
then manifest their proper Colours. These things have been prov'd, and
the sum of all this amounts to the Proposition here to be proved. For if
the Sun's Light is mix'd of several sorts of Rays, each of which have
originally their several Refrangibilities and colorific Qualities, and
notwithstanding their Refractions and Reflexions, and their various
Separations or Mixtures, keep those their original Properties
perpetually the same without alteration; then all the Colours in the
World must be such as constantly ought to arise from the original
colorific qualities of the Rays whereof the Lights consist by which
those Colours are seen. And therefore if the reason of any Colour
whatever be required, we have nothing else to do than to consider how
the Rays in the Sun's Light have by Reflexions or Refractions, or other
causes, been parted from one another, or mixed together; or otherwise to
find out what sorts of Rays are in the Light by which that Colour is
made, and in what Proportion; and then by the last Problem to learn the
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