to seek for any other way in which the movement of Light is
successively communicated, one will find none which agrees better,
with uniform progression, as seems to be necessary, than the property
of springiness; because if this movement should grow slower in
proportion as it is shared over a greater quantity of matter, in
moving away from the source of the light, it could not conserve this
great velocity over great distances. But by supposing springiness in
the ethereal matter, its particles will have the property of equally
rapid restitution whether they are pushed strongly or feebly; and thus
the propagation of Light will always go on with an equal velocity.

[Illustration]

And it must be known that although the particles of the ether are not
ranged thus in straight lines, as in our row of spheres, but
confusedly, so that one of them touches several others, this does not
hinder them from transmitting their movement and from spreading it
always forward. As to this it is to be remarked that there is a law of
motion serving for this propagation, and verifiable by experiment. It
is that when a sphere, such as A here, touches several other similar
spheres CCC, if it is struck by another sphere B in such a way as to
exert an impulse against all the spheres CCC which touch it, it
transmits to them the whole of its movement, and remains after that
motionless like the sphere B. And without supposing that the ethereal
particles are of spherical form (for I see indeed no need to suppose
them so) one may well understand that this property of communicating
an impulse does not fail to contribute to the aforesaid propagation
of movement.

Equality of size seems to be more necessary, because otherwise there