intermediate Bignesses answering
to the Colour, that is, greater in yellow than in green, and greater in
green than in blue. And hence I knew, that when the Speculum was
illuminated with white Light, the red and yellow on the outside of the
Rings were produced by the least refrangible Rays, and the blue and
violet by the most refrangible, and that the Colours of each Ring spread
into the Colours of the neighbouring Rings on either side, after the
manner explain'd in the first and second Part of this Book, and by
mixing diluted one another so that they could not be distinguish'd,
unless near the Center where they were least mix'd. For in this
Observation I could see the Rings more distinctly, and to a greater
Number than before, being able in the yellow Light to number eight or
nine of them, besides a faint shadow of a tenth. To satisfy my self how
much the Colours of the several Rings spread into one another, I
measured the Diameters of the second and third Rings, and found them
when made by the Confine of the red and orange to be to the same
Diameters when made by the Confine of blue and indigo, as 9 to 8, or
thereabouts. For it was hard to determine this Proportion accurately.
Also the Circles made successively by the red, yellow, and green,
differ'd more from one another than those made successively by the
green, blue, and indigo. For the Circle made by the violet was too dark
to be seen. To carry on the Computation, let us therefore suppose that
the Differences of the Diameters of the Circles made by the outmost red,
the Confine of red and orange, the Confine of orange and yellow, the
Confine of yellow and green, the Confine of green and blue, the Confine
of blue and indigo, the Confine of indigo and violet, and outmost
violet, are in proportion as the Differences of the Lengths of a
Monochord which sound the Tones in an Eight; _sol_, _la_, _fa_, _sol_,
_la_, _mi_, _fa_, _sol_, that is, as the Numbers 1/9, 1/18, 1/12, 1/12,
2/27, 1/27, 1/18. And if the Diameter of the Circle made by the Confine
of red and orange be 9A, and that of the Circle made by the Confine of
blue and indigo be 8A as above; their difference 9A-8A will be to the
difference of the Diameters of the Circles made by the outmost red, and
by the Confine of red and orange, as 1/18 + 1/12 + 1/12 + 2/27 to 1/9,
that is as 8/27 to 1/9, or 8 to 3, and to the difference of the Circles
made by the outmost violet, and by the Confine of blue and indigo, as
1/1