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 become white.
With any Obstacle, let all the Light be now stopp'd which passes through
any one Interval of the Teeth, so that the Range of Colours which comes
from thence may be taken away, and you will see the Light of the rest of
the Ranges to be expanded into the Place of the Range taken away, and
there to be coloured. Let the intercepted Range pass on as before, and
its Colours falling upon the Colours of the other Ranges, and mixing
with them, will restore the Whiteness.
Let the Paper 2D 2E be now very much inclined to the Rays, so that the
most refrangible Rays may be more copiously reflected than the rest, and
the white Colour of the Paper through the Excess of those Rays will be
changed into blue and violet. Let the Paper be as much inclined the
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