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The purpose of this paper is to derive the photon spin and to deduce itsproperties from a pair of quantum equations for the photon. To this end,Darwin's equations are reinterpreted so as to meet the need of the quantummechanics of the photon. It is found that the photon wavefunction transformsunder Lorentz transformation as a spinor. The relativistic nature of the photonis expressed through a constraint equation on the wavefunction in such a waythat the wave equation, which takes on the form of the Schr\"{o}dingerequation, is not Lorentz covariant unless the constraint equation is taken intoaccount. The wave equation predicts the existence of a kind of spin, anintrinsic degree of freedom. But the constraint equation makes the spinnonlocal in the sense that no unique local density exists for the spin inposition space. The nonlocality of the photon spin is a reflection of thenonlocality of the photon itself.

Spinor wave equation, relativistic condition, and nonlocality of photon spin