that although in the 4th and 18th Observations there appear no more than
eight or nine of those Rings, yet there are really a far greater number,
which so much interfere and mingle with one another, as after those
eight or nine revolutions to dilute one another wholly, and constitute
an even and sensibly uniform whiteness. And consequently that whiteness
must be allow'd a mixture of all Colours, and the Light which conveys it
to the Eye must be a mixture of Rays endued with all those Colours.
But farther; by the 24th Observation it appears, that there is a
constant relation between Colours and Refrangibility; the most
refrangible Rays being violet, the least refrangible red, and those of
intermediate Colours having proportionably intermediate degrees of
Refrangibility. And by the 13th, 14th, and 15th Observations, compared
with the 4th or 18th there appears to be the same constant relation
between Colour and Reflexibility; the violet being in like circumstances
reflected at least thicknesses of any thin Plate or Bubble, the red at
greatest thicknesses, and the intermediate Colours at intermediate
thicknesses. Whence it follows, that the colorifick Dispositions of
Rays are also connate with them, and immutable; and by consequence, that
all the Productions and Appearances of Colours in the World are derived,
not from any physical Change caused in Light by Refraction or Reflexion,
but only from the various Mixtures or Separations of Rays, by virtue of
their different Refrangibility or Reflexibility. And in this respect the
Science of Colours becomes a Speculation as truly mathematical as any
other part of Opticks. I mean, so far as they depend on the Nature of
Light, and are not produced or alter'd by the Power of Imagination, or
by striking or pressing the Eye.
THE
SECOND BOOK
OF
OPTICKS
137