A Green's function G for ∇² + κ² is interpreted essentially as a Laplace transform of a Green's function H for ∇² ‐ ∂/∂t. The Laplace integral is evaluated by selecting a mixing parameter T and representing H by rays in (0, T) and modes in (T, ∞). This procedure enables one to systematize, simplify, and extend the scope of the technique originated by Ewald (1916). As an illustration, G for a parallel‐plate wave guide is detailed.