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 _Traité de la Lumière_, 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 consideration to account for such phenomena as the refraction and reflection of light, but, as we have seen in Art. 46, such a property as density is inconceivable in connection with a medium which is neither atomic and possesses no mass. On the assumption, however, of an atomic and gravitative Aether, the difficulty is at once solved, and the density of the Aether, and different degrees of density are at once placed upon a logical and philosophical basis. So that in relation to the elasticity and density of the Aether upon which the transmission and reflection of wave motion depend, an atomic and gravitative Aether establishes and confirms the undulatory theory.