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 _Phænomena_ 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
aliâ contrariâ (refractione) non destruatur: Nam experientia docet si
superficies _NM_ & _NP_ (nempe refringentes) Parallelæ 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 _intermediate_
reflection shall be the same as if without any reflection at all the Ray
had been twice refracted by two Parallel surfaces.

And that this is true, not onely in one, but in every Ray that goes to the
constitution of the Primary Iris; nay, in every Ray, that suffers only two
refractions, and one reflection, by the surface of the round body, we shall