propagated or conveyed from place to place is energy, or
motion. If a stone is thrown into water, a series of concentric circles
of waves are generated, which spread out with increasing size, but
decreasing power or motion, regularly on all sides. The water, however,
does not move away from the generating source. There is a motion of the
water, but it is simply a wave motion, so that the propagation of a wave
is the propagation of motion, rather than the transference of the actual
water which constitutes the wave. In the case of sound waves, we have
again an illustration of the same principle. For example, suppose we
strike a bell, and so set the particles of that bell in a state of
vibration. These vibrations give the air in contact with the bell a
forward movement, and then, owing to the elasticity and inertia of the
air, a backward movement is set up, with the result that a series of
waves are set in motion from the bell on every side, which gradually
diminish in intensity the farther they recede from the generating body.
According to the wave theory, therefore, we have to picture all heated
and luminous bodies in a state of vibration, and the atoms of such
luminous bodies imparting the vibrations to the atoms of the Aether, in
the same way that the atoms of a bell impart their vibrations to the
atoms of the air in contact with it. These vibrations are then
propagated through the Aether in waves, which, entering the eye, impinge
or strike upon the retina at the back of the eye, and being transmitted
to the brain give rise to the sensation of sight. It must not be
forgotten that the waves of Aether, as pointed out in Art. 64 in
relation to heat, really form spherical shells which radiate out in all
directions from the central body which gives rise to them. Thus it can
be seen, that all points in the spherical wave which are at equal
distances from the vibratory or luminous body, must possess the same
intensity, and possess equal lighting powers. Light, therefore, like
heat, is due to a periodic wave motion set up in the Aether by the
vibrating atomic motion of heated or luminous bodies. It must be also
noticed, that if we could see the air through which the sound waves are
passing, we should see that each atom or particle of the atmosphere was
vibrating to and fro in the direction of propagation. If, however, we
could see an atom of Aether in vibration, accepting the principle that
Aether is atomic, we should see that