spectrum S will be contracted more and more, and at
length vanish, that Spectrum S becoming perfectly round and white; and
if you recede yet farther, the Colours will emerge again, but in a
contrary Order. Now that Spectrum S appears white in that Case, when the
Rays of several sorts which converge from the several Parts of the Image
PT, to the Prism _abc_, are so refracted unequally by it, that in their
Passage from the Prism to the Eye they may diverge from one and the same
Point of the Spectrum S, and so fall afterwards upon one and the same
Point in the bottom of the Eye, and there be mingled.

[Illustration: FIG. 7.]

[Illustration: FIG. 8.]

And farther, if the Comb be here made use of, by whose Teeth the Colours
at the Image PT may be successively intercepted; the Spectrum S, when
the Comb is moved slowly, will be perpetually tinged with successive
Colours: But when by accelerating the Motion of the Comb, the Succession
of the Colours is so quick that they cannot be severally seen, that
Spectrum S, by a confused and mix'd Sensation of them all, will appear
white.

_Exper._ 12. The Sun shining through a large Prism ABC [in _Fig._ 9.]
upon a Comb XY, placed immediately behind the Prism, his Light which
passed through the Interstices of the Teeth fell upon a white Paper DE.
The Breadths of the Teeth were equal to their Interstices, and seven
Teeth together with their Interstices took up an Inch in Breadth. Now,
when the Paper was about two or three Inches distant from the Comb, the
Light which passed through its several Interstices painted so many
Ranges of Colours, _kl_, _mn_, _op_, _qr_, &c. which were parallel to
one another, and contiguous, and without any Mixture of white. And these
Ranges of Colours, if the Comb was moved continually up and down with a
reciprocal Motion, ascended and descended in the Paper, and when the
Motion of the Comb was so quick, that the Colours could not be
distinguished from one another, the whole Paper by their Confusion and
Mixture in the Sensorium appeared white.

[Illustration: FIG. 9.]

Let the Comb now rest, and let the Paper be removed farther from the
Prism, and the several Ranges of Colours will be dilated and expanded
into one another more and more, and by mixing their Colours will dilute
one another, and at length, when the distance of the Paper from the Comb
is about a Foot, or a little more (suppose in the Place 2D 2E) they will
so far dilute one another, as to be