Second,—by a transfer of the energy from part to part of an intervening medium, each part as it receives the energy, transmitting it at once to the parts around it, no part undergoing more than a slight displacement from its normal position. This mode of transmission constitutes wave motion. The source imparts its energy to the particles of the medium near it. Each of those particles transfers its energy to the particles all around it. Each of these particles in turn transfers its energy to the particles around it, and so on through the medium. It is plain that there are here no such things as genuine rays. As the energy is transferred from particle to particle, each in turn becomes a centre of disturbance transmitting its motion in all directions. It is only because the movements transmitted from different points annul one another except along certain lines, that we have apparent straight lines of transmission, and, therefore, fairly sharp shadows. But shadows produced by wave transmissions are never absolutely sharp. The wave movement is always propagated to some extent within the boundary of the geometrical shadow, less as the wave lengths are shorter. With sound waves whose lengths are measured in inches or feet, the penetration into the shadow is considerable. With light waves 1/37000 to 1/70000 of an inch in length, the penetration into the shadow is very small and requires specially arranged apparatus to show that it exists. This penetration into the geometrical shadow is characteristic of energy propagated by wave motion, and if the fact of such penetration can be demonstrated, it is conclusive proof of propagation by waves. Another characteristic of wave motion is found in the phenomena of _interference_. This is the mutual effect of two wave systems, which, when meeting at a given point, may strengthen or annul each other according to the conditions under which they meet. Either of those characteristics should enable us to distinguish between propagation by wave motion and by projected particles. But when wave lengths are very short and radiations feeble, the tests are not easy to apply. Again, a wave is in general propagated with different velocities in different media. This causes a deflection or deformation of the wave as it passes from one medium into another, and results in _refraction_, as in the cases of light and sound. Absence of refraction would be strong though not conclusive evidence against a wave theory of propagation.