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 _Phaenomena_ 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 aetherei_,
which are the particles which he supposes to constitute the _Pellucid
medium_, But somewhat else, perhaps what we have lately supposed, and shall
by and by further prosecute and explain.
But, First I shall crave leave to propound some other difficulties of his,
notwithstanding exceedingly ingenious _Hypothesis_, which I plainly confess
to me seem such; and those are,
First, if that light be (as is affirmed, _Diopt._ cap. 1. Sec. 8.) not so
properly a motion, as an action or propension to motion, I cannot conceive
how the eye can come to be sensible of the _verticity_ of a _Globule_,
which is generated in a drop of Rain, perhaps a mile off from it.
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