This page has been validated.
It is known that the differential equations for the oscillations of an elastic incompressible medium read:

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.