  The Foldy-Wouthuysen iterative diagonalization technique is applied to the
Helmholtz equation to obtain a Hamiltonian description of the propagation of a
monochromatic quasiparaxial light beam through a system in which the refractive
index $n(x,y,z)$ varies about a background value $n_0$ such that
$|n(x,y,z)-n_0| \ll n_0$. This technique is presented as an alternative to the
conventional method of series expansion of the radical. Besides reproducing all
the traditional quasiparaxial terms, this method leads to additional terms in
the optical Hamiltonian.
