each aetherial atom is not
vibrating in the direction of propagation, but across the line in which
the wave is travelling. Thus the vibration of the air is said to be
longitudinal, but the vibrations of the Aether are transversal. An
illustration of the transverse motion of a light wave may be obtained by
taking a rope and imparting to it a series of undulations by shaking it
up and down, when it will be observed that the wave motion of the rope
is transverse to the straight line in which it is propagated. The
physical explanation of the transverse vibration of light will be dealt
with in a subsequent article.

Now the question suggests itself to our mind, as to what effect the
atomicity of the Aether has upon the undulatory theory of light. Does it
establish it upon a firmer basis, or does it in any way destroy its
truth as a theory? I venture to think that the atomicity of the Aether
in no sense destroys any part of the undulatory theory of light, but
rather tends to confirm and establish it upon a logical and
philosophical basis.

For instance, as has been pointed out in Art. 47, in order for the
undulatory theory to have any existence at all, it is essential that the
Aether should possess the property of elasticity. But how the Aether
possessed the property of elasticity while at the same time it was
frictionless, and therefore possessed no mass, has been a problem that
has taxed the ingenuity and resources of scientists for a century past,
and up to the present is a problem which still remains unsolved. Now,
however, with our atomic Aether, it is just as easy to conceive Aether
transmitting a wave as it is for air to transmit sound waves, or water
to transmit water waves.

Tyndall, in his _Lectures on Light_, seems to have appreciated the
difficulty, and to avoid confusion, again and again refers to a
_particle_ of Aether. While Huyghens himself in speculating upon the
elasticity of the Aether in his _Traite de la Lumiere_, 1678, makes a
suggestion as to its origin, which practically amounts to the fact that
the aetherial atom which gives rise to this elasticity is the core or
centre of a vortex ring. Thus it can be seen that the elasticity of the
Aether, so essential to the undulatory theory, is a problem that cannot
be solved apart from recognizing the hypothesis of an atomic Aether.

Then, again, in the undulatory theory of light, the density of the
Aether around molecules of bodies has to be taken into cons