thick thick piece of the same substance which had a flat surface,
slightly soiled with breath or in some other way, there remained round
marks, of smaller or larger size according as the blow had been weak
or strong. This makes it evident that these substances yield where
they meet, and spring back: and for this time must be required.

Now in applying this kind of movement to that which produces Light
there is nothing to hinder us from estimating the particles of the
ether to be of a substance as nearly approaching to perfect hardness
and possessing a springiness as prompt as we choose. It is not
necessary to examine here the causes of this hardness, or of that
springiness, the consideration of which would lead us too far from our
subject. I will say, however, in passing that we may conceive that the
particles of the ether, notwithstanding their smallness, are in turn
composed of other parts and that their springiness consists in the
very rapid movement of a subtle matter which penetrates them from
every side and constrains their structure to assume such a disposition
as to give to this fluid matter the most overt and easy passage
possible. This accords with the explanation which Mr. Des Cartes gives
for the spring, though I do not, like him, suppose the pores to be in
the form of round hollow canals. And it must not be thought that in
this there is anything absurd or impossible, it being on the contrary
quite credible that it is this infinite series of different sizes of
corpuscles, having different degrees of velocity, of which Nature
makes use to produce so many marvellous effects.

But though we shall ignore the true cause of springiness we still see
that there are many bodies which possess this property; and thus there
is nothing strange in supposing that it exists also in little
invisible bodies like the particles of the Ether. Also if one wishes