, proceeds partly from the wideness of the hole through
which the Rays pass, whereby the Rays from several parts of the luminous
body, fall upon many of the same parts between c and f as is more manifest
by the Figure: And partly also from the nature of the refraction it self,
for the vividness or strength of the two terminating colours, arising
chiefly as we have seen, from the very great difference that is betwixt the
outsides of those _oblique undulations_ & the dark Rays circumambient, and
that disparity betwixt the _approximate_ Rays, decaying gradually: the
further inward toward the middle of the luminous body they are remov'd, the
more must the colour approach to a white or an undisturbed light.

Upon the calculation of the refraction and reflection from a Ball of Water
or Glass, we have much the same _Phaenomena_, namely, an _obliquity_ of the
undulation in the same manner as we have found it here. Which, because it
is very much to our present purpose, and affords such an _Instancia
crucis_, as no one that I know has hitherto taken notice of, I shall
further examine. For it does very plainly and positively distinguish, and
shew, which of the two _Hypotheses_, either the _Cartesian_ or this is to
be followed, by affording a generation of all the colors in the Rainbow,
where according to the _Cartesian Principles_ there should be none at all
generated. And secondly, by affording an instance that does more closely
confine the cause of these _Phaenomena_ of colours to this present
_Hypothesis_.

And first, for the _Cartesian_, we have this to object against it, That
whereas he says (_Meteorum Cap. 8. Sect. 5._) _Sed judicabam unicam
(refractione scilicet) ad minimum requiri, & quidem talem ut ejus effectus
alia contraria (refractione) non destruatur: Nam experientia docet si
superficies _NM_ & _NP_ (nempe refringentes) Parallelae forent, radios
tantundem per alteram iterum erectos quantum per unam frangerentur, nullos
colores depicturos_; This Principle of his holds true indeed in a prisme
where the refracting surfaces are plain, but is contradicted by the Ball or
Cylinder, whether of Water Or Glass, where the refracting surfaces are
Orbicular or Cylindrical. For if we examine the passage of any _Globule_ or
Ray of the primary _Iris_, we shall find it to pass out of the Ball or
Cylinder again, with the same inclination and refraction that it enter'd in
withall, and that that last refraction by means of the _intermedi