rule.
Generally the oscillations in the sub-aether are too rapid for us to detect directly; their frequency reaches the plane of ordinary experience by affecting the speed of propagation, because the speed depends (as already stated) on the wave-length or frequency. Calling the frequency
, the equation expressing the law of propagation of the ripples will contain a term in
. There will be another term expressing the modification caused by the “field of force” emanating from the bodies present in the neighbourhood. This can be treated as a kind of spurious
, since it emerges into our gross experience by the same method that