be entirely inapplicable. Historically, in 1849, Sir George Stokes stated his optical reversion principle without attending to polarization. Like the principles of thermodynamics, this principle is reliable enough to use as a check on the correct performance of experiments, in contrast with the usual situation in which the experiments are tests of a proposed law. The simplest statement of the principle is if I can see you, then you can see me. The principle was used by Gustav Kirchhoff in his derivation of his law of thermal radiation and by Max Planck in his analysis of his law of thermal radiation. For ray-tracing global illumination algorithms, incoming and outgoing light can be considered as reversals of each other, without affecting the bidirectional reflectance distribution function (BRDF) outcome. Green's reciprocity Whereas the above reciprocity theorems were for oscillating fields, Green's reciprocity is an analogous theorem for electrostatics with a fixed distribution of electric charge (Panofsky and Phillips, 1962). In particular, let denote the electric potential resulting from a total charge density . The electric potential satisfies Poisson's equation, , where is the vacuum permittivity. Similarly, let denote the electric potential resulting from a total charge density , satisfying . In both cases, we assume that the charge distributions are localized, so that the potentials can be chosen to go to zero at infinity. Then, Green's reciprocity theorem states that, for integrals over all space: This theorem is easily proven from Green's second identity. Equivalently, it is the statement that i.e. that is a Hermitian operator (as follows by integrating by parts twice). See also Surface equivalence principle References L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media (Addison-Wesley: Reading, MA, 1960). §89. Ronold W. P. King, Fundamental Electromagnetic Theory (Dover: New York, 1963). §IV.21. C. Altman and K. Such, Reciprocity, Spatial Mapping and Time Reversal in Electromagnetics (Kluwer: Dordrecht, 1991). H. A. Lorentz, "The theorem of Poynting concerning the energy in the electromagnetic field and two general propositions concerning the propagation of light," Amsterdammer Akademie der Wetenschappen 4 p. 176 (1896). R. J. Potton, "Reciprocity in optics," Reports on Progress in Physics 67, 717-754 (2004). (A review article on the history of this topic.) J. R. Carson, "A generalization of reciprocal theorem," Bell System Technical Journal 3 (3), 393-399 (1924). J. R. Carson, "The reciprocal energy theorem," ibid. 9 (4), 325-331 (1930). Ya. N. Feld, "On the quadratic lemma