a
continual Succession. But if the Impressions follow one another so
quickly, that they cannot be severally perceived, there ariseth out of
them all one common Sensation, which is neither of this Colour alone nor
of that alone, but hath it self indifferently to 'em all, and this is a
Sensation of Whiteness. By the Quickness of the Successions, the
Impressions of the several Colours are confounded in the Sensorium, and
out of that Confusion ariseth a mix'd Sensation. If a burning Coal be
nimbly moved round in a Circle with Gyrations continually repeated, the
whole Circle will appear like Fire; the reason of which is, that the
Sensation of the Coal in the several Places of that Circle remains
impress'd on the Sensorium, until the Coal return again to the same
Place. And so in a quick Consecution of the Colours the Impression of
every Colour remains in the Sensorium, until a Revolution of all the
Colours be compleated, and that first Colour return again. The
Impressions therefore of all the successive Colours are at once in the
Sensorium, and jointly stir up a Sensation of them all; and so it is
manifest by this Experiment, that the commix'd Impressions of all the
Colours do stir up and beget a Sensation of white, that is, that
Whiteness is compounded of all the Colours.

And if the Comb be now taken away, that all the Colours may at once pass
from the Lens to the Paper, and be there intermixed, and together
reflected thence to the Spectator's Eyes; their Impressions on the
Sensorium being now more subtilly and perfectly commixed there, ought
much more to stir up a Sensation of Whiteness.

You may instead of the Lens use two Prisms HIK and LMN, which by
refracting the coloured Light the contrary Way to that of the first
Refraction, may make the diverging Rays converge and meet again in G, as
you see represented in the seventh Figure. For where they meet and mix,
they will compose a white Light, as when a Lens is used.

_Exper._ 11. Let the Sun's coloured Image PT [in _Fig._ 8.] fall upon
the Wall of a dark Chamber, as in the third Experiment of the first
Book, and let the same be viewed through a Prism _abc_, held parallel to
the Prism ABC, by whose Refraction that Image was made, and let it now
appear lower than before, suppose in the Place S over-against the red
Colour T. And if you go near to the Image PT, the Spectrum S will appear
oblong and coloured like the Image PT; but if you recede from it, the
Colours of the