presently see most evident, if we repeat the _Cartesian Scheme_, mentioned
in the tenth _Section_ of the eighth _Chapter_ of his _Meteors_, where
EFKNP in the third Figure[9] is one of the Rays of the Primary Iris, twice
refracted at F and N, and once reflected at K by the surface of the
Water-ball. For, first it is evident, that KF and KN are equal, because KN
being the reflected part of KF they have both the same inclination on the
surface K that is the angles FKT, and NKV made by the two Rays and the
Tangent of K are equal, which is evident by the Laws of reflection; whence
it will follow also, that KN has the same inclination on the surface N, or
the Tangent of it XN that the Ray KF has to the surface F, or the Tangent
of it FY, whence it must necessarily follow, that the refractions at F and
N are equal, that is, KFE and KNP are equal. Now, that the surface N is by
the reflection at K made parallel to the surface at F, is evident from the
principles of reflection; for reflection being nothing but an inverting of
the Rays, if we re-invert the Ray KNP, and make the same inclinations below
the line TKV that it has above, it will be most evident, that KH the
inverse of KN will be the continuation of the line FK, and that LHI the
inverse of OX is parallel to FY. And HM the inverse of NP is Parallel to EF
for the angle KHI is equal to KNO which is equal to KFY, and the angle KHM
is equal to KNP which is equal to KFE which was to be prov'd.

So that according to the above mentioned _Cartesian_ principles there
should be generated no colour at all in a Ball of Water or Glass by two
refractions and one reflection, which does hold most true indeed, if the
surfaces be plain, as may be experimented with any kind of prisme where the
two refracting surfaces are equally inclin'd to the reflecting; but in this
the _Phænomena_ are quite otherwise.

The cause therefore of the generation of colour must not be what _Des
Cartes_ assigns, namely, a certain _rotation_ of the _Globuli ætherei_,