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
ought to be some reflexion of movement backwards when it passes from a
smaller particle to a larger one, according to the Laws of Percussion
which I published some years ago.
However, one will see hereafter that we have to suppose such an
equality not so much as a necessity for the propagation of light as
for rendering that propagation easier and more powerful; for it is not
beyond the limits of probability that the particles of the ether have
been made equal for a purpose so important as that of light, at least
in that vast space which is beyond the region of atmosphere and which
seems to serve only to transmit the light of the Sun and the Stars.
I have then shown in what manner one may conceive Light to spread
successively, by spherical waves, and how it is possible that this
spreading is accomplished with as great a velocity as that which
experiments and celestial observations demand. Whence it may be
further remarked that although the particles are 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.
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