The following two modes of generation of the wave-surface are pretty generally known.

(a) A given ellipsoid (surface of elasticity) is cut by any central plane π along an ellipse of semiaxes λ1 and λ2. If π varies, the two pairs of planes π∲ parallel to π at distances k2/λ1, k2/λ2 (k = constant) from it envelope the wave-surface W1 represented by the tangential equation

if the tangential coordinates u1 depend on Σux +1 = 0.