Opticks 1717 - Up

Then let another Priiili 5) I3 G de& parallel to
the former 9 be placed at X ,’ to refia& that
wllil:e Light upwards towards Y. Let the refiatking
Angles of the l-‘rilills, and their diitances
from t$e Lens be equal,- i;, that the Rays which
cdnverged from the Lens towards X, and without
Refraktion, would there have crofikd and
diverged again, may by the Rcfrafiion of the
fecond priiin be reduced into I’nrakfifk and
diverge no more., For then thok Ra s will recompolfe
a beam of white Light X c ‘. If the
refracrting Angle of either Priiin be the bigger,
that Prifin muff be So much the nearer to the
IL&. You wi1.l know when the Prifms and the
Lens are well fet together, by obkrving if the
beam of Light XY which comes out of the j‘econd’Prifm
be perfeAly white to the very edges
of the Light; and at all diitances from the Priiin
continue perfeAly and totally white like a bcatn
of the Sun’s Light. For till” this happens, the
pofition of the Prifms and Lens to one mother
muit be correkkd, and then if by the help of a
long beam of Wood ,’ as is roprefentcd in the,
Figure, or by a Tube, or fame other frrch Infirument
made for that purpol’e, they be made
MI in that fituation, you may try all the fame
Expenrimentg in this compounded beam ofLight
X Y, which have been made in the Sun’s dire&
Light. For this compounded beam df Light
has the fame appearance, and is cndow’d with
all the fame Properties with a dire& beam of
the’Sun’s Light, fo far as my Obfervatiol~ reaches,
And in trying Experiments in this beam yo’u
may by fiopping any of the Colours p, g, r, I
ad