Book excerpt: The reports investigates the propagation of light in the visible region through a crystalline medium. In Part I, the objective is to determine if the anisotropic arrangement of ordinary (isotropic) dipoles at the nodal points of an orthorhombic lattice would account for the existence of double refraction. Some features of the traditional 'theory of dispersion' are disclosed and clarified. In part II, a crystalline medium is considered as filling a half- space and having a plane boundary at z=0. A plane optical wave is incident on this medium. Because of the linearity of the equations it has to be superimposed on the field originating in the crystal. It is shown that this incident optical wave is actually prevented from entering the crystal because of the modification produced in the field of the crystal by the introduction of a boundary. Material Added 1970. The conclusion that the incident optical field cannot penetrate the crystal boundary, together with a similar conclusion in a paper by Oseen, is the basis of the Ewald-Oseen Extinction Theorem.