at Place PT, and when the Prism is restored to its Place again, so
that its Colours may there fall upon the Colours of the other, the
Mixture of them both will restore the Whiteness.

This Experiment succeeds also, as I have tried, when the Angle _b_ of
the lower Prism, is a little greater than the Angle B of the upper, and
between the interior Angles B and _c_, there intercedes some Space B_c_,
as is represented in the Figure, and the refracting Planes BC and _bc_,
are neither in Directum, nor parallel to one another. For there is
nothing more requisite to the Success of this Experiment, than that the
Rays of all sorts may be uniformly mixed upon the Paper in the Place PT.
If the most refrangible Rays coming from the superior Prism take up all
the Space from M to P, the Rays of the same sort which come from the
inferior Prism ought to begin at P, and take up all the rest of the
Space from thence towards N. If the least refrangible Rays coming from
the superior Prism take up the Space MT, the Rays of the same kind which
come from the other Prism ought to begin at T, and take up the
remaining Space TN. If one sort of the Rays which have intermediate
Degrees of Refrangibility, and come from the superior Prism be extended
through the Space MQ, and another sort of those Rays through the Space
MR, and a third sort of them through the Space MS, the same sorts of
Rays coming from the lower Prism, ought to illuminate the remaining
Spaces QN, RN, SN, respectively. And the same is to be understood of all
the other sorts of Rays. For thus the Rays of every sort will be
scattered uniformly and evenly through the whole Space MN, and so being
every where mix'd in the same Proportion, they must every where produce
the same Colour. And therefore, since by this Mixture they produce white
in the Exterior Spaces MP and TN, they must also produce white in the
Interior Space PT. This is the reason of the Composition by which
Whiteness was produced in this Experiment, and by what other way soever
I made the like Composition, the Result was Whiteness.

Lastly, If with the Teeth of a Comb of a due Size, the coloured Lights
of the two Prisms which fall upon the Space PT be alternately
intercepted, that Space PT, when the Motion of the Comb is slow, will
always appear coloured, but by accelerating the Motion of the Comb so
much that the successive Colours cannot be distinguished from one
another, it will appear white.

_Exper._ 14. Hitherto I