re supposed to be in
continual movement (for there are many reasons for this) the
successive propagation of the waves cannot be hindered by this;
because the propagation consists nowise in the transport of those
particles but merely in a small agitation which they cannot help
communicating to those surrounding, notwithstanding any movement which
may act on them causing them to be changing positions amongst
themselves.

But we must consider still more particularly the origin of these
waves, and the manner in which they spread. And, first, it follows
from what has been said on the production of Light, that each little
region of a luminous body, such as the Sun, a candle, or a burning
coal, generates its own waves of which that region is the centre. Thus
in the flame of a candle, having distinguished the points A, B, C,
concentric circles described about each of these points represent the
waves which come from them. And one must imagine the same about every
point of the surface and of the part within the flame.

[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