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 these waves can be
infinitely feeble only as compared with the wave DCF, to the
composition of which all the others contribute by the part of their
surface which is most distant from the centre A.
One sees, in addition, that the wave DCF is determined by the
distance attained in a certain space of time by the movement which
started from the point A; there being no movement beyond this wave,
though there will be in the space which it encloses, namely in parts
of the particular waves, those parts which do not touch the sphere
DCF. And all this ought not to seem fraught with too much minuteness
or subtlety, since we shall see in the sequel that all the properties
of Light, and everything pertaining to its reflexion and its
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