e Light XY. If the refracting Angle of either Prism be the
bigger, that Prism must be so much the nearer to the Lens. You will know
when the Prisms and the Lens are well set together, by observing if the
beam of Light XY, which comes out of the second Prism be perfectly white
to the very edges of the Light, and at all distances from the Prism
continue perfectly and totally white like a beam of the Sun's Light. For
till this happens, the Position of the Prisms and Lens to one another
must be corrected; and then if by the help of a long beam of Wood, as is
represented in the Figure, or by a Tube, or some other such Instrument,
made for that Purpose, they be made fast in that Situation, you may try
all the same Experiments in this compounded beam of Light XY, which have
been made in the Sun's direct Light. For this compounded beam of Light
has the same appearance, and is endow'd with all the same Properties
with a direct beam of the Sun's Light, so far as my Observation reaches.
And in trying Experiments in this beam you may by stopping any of the
Colours, _p_, _q_, _r_, _s_, and _t_, at the Lens, see how the Colours
produced in the Experiments are no other than those which the Rays had
at the Lens before they entered the Composition of this Beam: And by
consequence, that they arise not from any new Modifications of the Light
by Refractions and Reflexions, but from the various Separations and
Mixtures of the Rays originally endow'd with their colour-making
Qualities.

So, for instance, having with a Lens 4-1/4 Inches broad, and two Prisms
on either hand 6-1/4 Feet distant from the Lens, made such a beam of
compounded Light; to examine the reason of the Colours made by Prisms, I
refracted this compounded beam of Light XY with another Prism HIK _kh_,
and thereby cast the usual Prismatick Colours PQRST upon the Paper LV
placed behind. And then by stopping any of the Colours _p_, _q_, _r_,
_s_, _t_, at the Lens, I found that the same Colour would vanish at the
Paper. So if the Purple _p_ was stopp'd at the Lens, the Purple P upon
the Paper would vanish, and the rest of the Colours would remain
unalter'd, unless perhaps the blue, so far as some purple latent in it
at the Lens might be separated from it by the following Refractions. And
so by intercepting the green upon the Lens, the green R upon the Paper
would vanish, and so of the rest; which plainly shews, that as the white
beam of Light XY was compounded of several Lights va