The reciprocity principle in lunar photometry

In order to ascertain the exact meaning of the optical reciprocity principle, stated by Helmholtz, some simple idealized experiments are considered. By then applying this principle to the case of a scatter- ing surface, we find a general reciprocity law which is independent of the nature of the surface considered. Definite relations must, therefore, exist between the brightness of two points on the moon, if the lunar surface at these points has identical properties; a photometric test for this identity is thus obtained. From the work of Opik, Fessenkoff, and Bennett examples are selected in which similar lunar formations are found to have similar surface layers, and other examples in which the surfaces are found to be differ- ent. The reciprocity principle restricts the possible forms of the general law of illumination of the lunar surface. Some of the laws thus far proposed are incompatible with it