In a paper, now nearly thirty years old, I applied Maxwell’s equations of the electro-magnetic field to investigate the disturbance produced by an obstacle upon plane waves of light which travel through a medium otherwise uniform, giving particular attention to the case where the properties of the obstacle differ but little from those of its surroundings. The difference may consist in a variation of K — the specific inductive capacity, or of μ — the magnetic capacity, or of both; but it was shown that the last supposition leads to results inconsistent with observation, and that the evidence favours the view that μ is to be treated as invariable. Denoting electric displacements by f, g, h , the primary wave was taken to be h0 = einteikx , (23) so that the direction of propagation is along x (negatively), and that of vibration parallel to z . ∆ μ being omitted, the electric displacements ( f1 , g1 , h1 ) in the scattered wave, so far as they depend upon the first power of ∆K, have at a great distance the values f1 , g1 , h1 = k2 KP/4 πr ( αγ / r2 , βγ / r2 , – α2 + β2 / r2 ), (35, 37, 38) in which P = ∭ h0 ∆K -1e-ikrdx dy dz . (36)