  The difficulties encountered up till now in the theory of identifying the
spin and orbital angular momentum of the photon stem from the approach of
dividing the angular momentum of the photon into spin and orbital parts. Here
we derive the spin of the photon from a set of two relativistic quantum
equations that was first cast from the free-space Maxwell equations by Darwin
in 1932. Much attention is focused on the nonlocal properties of the photon
spin that are determined by the relativistic quantum constraint, one of the
so-called Darwin equations. Meanwhile, we point out that for the Darwin
equations to describe the quantum motion of the photon, the upper and lower
parts of the wavefunction cannot be the electric and magnetic fields as Darwin
prescribed. Their nonlocal relations are investigated. The Lorentz covariance
of the Darwin equations is also proven, to the best of our knowledge, for the
first time.
