bodies. This
is assuredly the mark of motion, at least in the true Philosophy, in
which one conceives the causes of all natural effects in terms of
mechanical motions. This, in my opinion, we must necessarily do, or
else renounce all hopes of ever comprehending anything in Physics.

And as, according to this Philosophy, one holds as certain that the
sensation of sight is excited only by the impression of some movement
of a kind of matter which acts on the nerves at the back of our eyes,
there is here yet one reason more for believing that light consists in
a movement of the matter which exists between us and the luminous
body.

Further, when one considers the extreme speed with which light spreads
on every side, and how, when it comes from different regions, even
from those directly opposite, the rays traverse one another without
hindrance, one may well understand that when we see a luminous object,
it cannot be by any transport of matter coming to us from this object,
in the way in which a shot or an arrow traverses the air; for
assuredly that would too greatly impugn these two properties of light,
especially the second of them. It is then in some other way that light
spreads; and that which can lead us to comprehend it is the knowledge
which we have of the spreading of Sound in the air.

We know that by means of the air, which is an invisible and impalpable
body, Sound spreads around the spot where it has been produced, by a
movement which is passed on successively from one part of the air to
another; and that the spreading of this movement, taking place equally
rapidly on all sides, ought to form spherical surfaces ever enlarging
and which strike our ears. Now there is no doubt at all that light
also comes from the luminous body to our eyes by some movement
impressed on the matter which is between the two; since, as we have
already seen, it cannot be by the transport of a body which passes
from one to the other. If, in addition, light takes time for its
passage--which we are now going to examine--it will follow that this
movement, impressed on the intervening matter, is successive; and
consequently it spreads, as Sound does, by spherical surfaces and
waves: for I call them waves from their resemblance to those which are
seen to be formed in water when a stone is thrown into it, and which
present a successive spreading as circles, though these arise from
another cause, and are only in a flat surface.

To see the