[Illustration]
But as the percussions at the centres of these waves possess no
regular succession, it must not be supposed that the waves themselves
follow one another at equal distances: and if the distances marked in
the figure appear to be such, it is rather to mark the progression of
one and the same wave at equal intervals of time than to represent
several of them issuing from one and the same centre.
After all, this prodigious quantity of waves which traverse one
another without confusion and without effacing one another must not be
deemed inconceivable; it being certain that one and the same particle
of matter can serve for many waves coming from different sides or even
from contrary directions, not only if it is struck by blows which
follow one another closely but even for those which act on it at the
same instant. It can do so because the spreading of the movement is
successive. This may be proved by the row of equal spheres of hard
matter, spoken of above. If against this row there are pushed from two
opposite sides at the same time two similar spheres A and D, one will
see each of them rebound with the same velocity which it had in
striking, yet the whole row will remain in its place, although the
movement has passed along its whole length twice over. And if these
contrary movements happen to meet one another at the middle sphere, B,
or at some other such as C, that sphere will yield and act as a 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
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