When one takes a number of spheres of equal size, made of some very
hard substance, and arranges them in a straight line, so that they
touch one another, one finds, on striking with a similar sphere
against the first of these spheres, that the motion passes as in an
instant to the last of them, which separates itself from the row,
without one's being able to perceive that the others have been
stirred. And even that one which was used to strike remains motionless
with them. Whence one sees that the movement passes with an extreme
velocity which is the greater, the greater the hardness of the
substance of the spheres.
But it is still certain that this progression of motion is not
instantaneous, but successive, and therefore must take time. For if
the movement, or the disposition to movement, if you will have it so,
did not pass successively through all these spheres, they would all
acquire the movement at the same time, and hence would all advance
together; which does not happen. For the last one leaves the whole row
and acquires the speed of the one which was pushed. Moreover there are
experiments which demonstrate that all the bodies which we reckon of
the hardest kind, such as quenched steel, glass, and agate, act as
springs and bend somehow, not only when extended as rods but also when
they are in the form of spheres or of other shapes. That is to say
they yield a little in themselves at the place where they are struck,
and immediately regain their former figure. For I have found that on
striking with a ball of glass or of agate against a large and quite
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
18