Image (suppose at _pt_) will appear upon it intensely coloured after the
manner above-explained, and that by receding from the Lens those Colours
will perpetually come towards one another, and by mixing more and more
dilute one another continually, until at length the Paper come to the
Focus G, where by a perfect mixture they will wholly vanish and be
converted into whiteness, the whole Light appearing now upon the Paper
like a little white Circle. And afterwards by receding farther from the
Lens, the Rays which before converged will now cross one another in the
Focus G, and diverge from thence, and thereby make the Colours to appear
again, but yet in a contrary order; suppose at [Greek: de], where the
red _t_ is now above which before was below, and the violet _p_ is below
which before was above.
Let us now stop the Paper at the Focus G, where the Light appears
totally white and circular, and let us consider its whiteness. I say,
that this is composed of the converging Colours. For if any of those
Colours be intercepted at the Lens, the whiteness will cease and
degenerate into that Colour which ariseth from the composition of the
other Colours which are not intercepted. And then if the intercepted
Colours be let pass and fall upon that compound Colour, they mix with
it, and by their mixture restore the whiteness. So if the violet, blue
and green be intercepted, the remaining yellow, orange and red will
compound upon the Paper an orange, and then if the intercepted Colours
be let pass, they will fall upon this compounded orange, and together
with it decompound a white. So also if the red and violet be
intercepted, the remaining yellow, green and blue, will compound a green
upon the Paper, and then the red and violet being let pass will fall
upon this green, and together with it decompound a white. And that in
this Composition of white the several Rays do not suffer any Change in
their colorific Qualities by acting upon one another, but are only
mixed, and by a mixture of their Colours produce white, may farther
appear by these Arguments.
[Illustration: FIG. 6.]
If the Paper be placed beyond the Focus G, suppose at [Greek: de], and
then the red Colour at the Lens be alternately intercepted, and let pass
again, the violet Colour on the Paper will not suffer any Change
thereby, as it ought to do if the several sorts of Rays acted upon one
another in the Focus G, where they cross. Neither will the red upon the
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