"A reciprocity theorem for the interaction of electromagnetic plane waves with a one-dimensional inhomogeneous slab"

In the above correspondence [ibid., vol. 51. p. 1268; September, 1963], Swift has gone through a rather elaborate analysis to show that the (horizontally and vertically polarized) plane wave transmission coefficients for a slab with continuously varying refractive index along the direction perpendicular to the interfaces are the same when the wave impinges on the slab at a given angle from above or from below, but that the reflection coefficients usually differ under the same circumstances. It may be worthwhile mentioning that these conclusions could have been deduced from a direct application of the conventional reciprocity theorem which is valid for homogeneous or inhomogeneous media.