o the Knowledge of the Nature of
_Reflection_ and _Refraction_. But the Person, that did most learnedly and
resolutely attack the said _Dioptricks_, was Monsieur _Fermat_, {393}
writing first about it to _Mersennus_, who soon communicated his Objections
to M. _Des-Cartes_, who failed not to return his Answer to them. But
_Fermat_ replied, and _Des-Cartes_ likewise; and after many reciprocations,
in which each party pretended to have the advantage, the matter rested;
until M. _Fermat_ taking occasion to write afresh of it to M. _De la
Chambre_, several years after _Des-Cartes_'s death, upon occasion of a
Book, written M. _De la Chambre_, _Of Light_; discoursed with this new
_Author_ after the same rate, as he had done before with _Des-Cartes_
himself, and seemed to invite some-body of his friends, to re-assume the
former contest. Whereupon M. _Clerselier_ and M. _Rohault_ took up the
Gantlet, to assert the Doctrine of the deceased Philosopher, exchanging
several Letters with M. _Fermat_, all inserted in this _Tome_, and serving
fully to instruct the Reader of this Difference, and withal to elucidate
many difficult points of the Subject of _Refractions_; especially of this
particular, _Whether the Motion of Light is more easily, and with more
expedition, perform'd through _dense_ Mediums, than _rare_._

Besides this, though one would think, Disputes had no place in _Geometry_,
since all proofs there, are as many Demonstrations; yet M. _Des-Cartes_
hath had several scufles touching that Science. As M. _Fermat_ had
assaulted his _Dioptricks_, so He reciprocally examined his Treatise _De
Maximis & Minimis_, pretending to have met with _Paralogismes_ in it. But
the Cause of M. _Fermat_ was learnedly pleaded for, by some of his Friends,
who took their turn to examine the Treatise of _Des-Carte_'s Geometry;
whereupon many _Letters_ were exchanged, to be found this Book, and
deserving to be considered; which doubtless the Curious would easily be
induced to do, if Copies of this Book were to be obtain'd here in
_England_, besides that one, which the _Publisher_ received from his
_Parisian_ Correspondent, and which affords him the opportunity of giving
this, though but Cursory, Account of it.

As to _Physicks_, there occur chiefly two Questions, learnedly treated of
in this _Volume_, though not without some heat between M. _Des-Cartes_ and
M. _Roberval_. The _one_ is, touching the Vibrations of Bodies suspended in
the Air, and the