spring
at both sides, and so will serve at the same instant to transmit these
two movements.

[Illustration]

But what may at first appear full strange and even incredible is that
the undulations produced by such small movements and corpuscles,
should spread to such immense distances; as for example from the Sun
or from the Stars to us. For the force of these waves must grow feeble
in proportion as they move away from their origin, so that the action
of each one in particular will without doubt become incapable of
making itself felt to our sight. But one will cease to be astonished
by considering how at a great distance from the luminous body an
infinitude of waves, though they have issued from different points of
this body, unite together in such a way that they sensibly compose one
single wave only, which, consequently, ought to have enough force to
make itself felt. Thus this infinite number of waves which originate
at the same instant from all points of a fixed star, big it may be as
the Sun, make practically only one single wave which may well have
force enough to produce an impression on our eyes. Moreover from each
luminous point there may come many thousands of waves in the smallest
imaginable time, by the frequent percussion of the corpuscles which
strike the Ether at these points: which further contributes to
rendering their action more sensible.

[Illustration]

There is the further consideration in the emanation of these waves,
that each particle of matter in which a wave spreads, ought not to
communicate its motion only to the next particle which is in the
straight line drawn from the luminous point, but that it also imparts
some of it necessarily to all the others which touch it and which
oppose themselves to its movement. So it arises that around each
particle there is made a wave of which that particle is the centre.
Thus if DCF is a wave emanating from the luminous point A, which is
its centre, the particle B, one of those comprised within the sphere
DCF, will have made its particular or partial wave KCL, which will
touch the wave DCF at C at the same moment that the principal wave
emanating from the point A has arrived at DCF; and it is clear that it
will be only the region C of the wave KCL which will touch the wave
DCF, to wit, that which is in the straight line drawn through AB.
Similarly the other particles of the sphere DCF, such as _bb_, _dd_,
etc., will each make its own wave. But each of