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It is known that the differential equations for the oscillations of an elastic medium are:

1) |

where ω is the propagation velocity of the oscillations - or more precisely the propagation velocity of plane waves with constant amplitude. It is presupposed that *u, v, w* fulfill the relation:

1') |

Now let *u = U, v = V, w = W* be solutions of these equations, which on a given surface adopt given values , , which depend on time, then we can say that these functions *U, V, W* represent the law by which the surface *f* = 0 is *illuminating*.