making Fresnelian convection co-efficient simply unity. Equations (1·21) and (2·21) may be obtained more simply from physical considerations. According to Heaviside and Hertz, the real seat of both electric and magnetic polarisation is the moving medium itself. Now at a point which is fixed with respect to the ether, the rate of change of electric polarisation is δD/δ_t_. Consider a slab of matter moving with velocity _u__{_x_} along the _x_-axis, then even in a stationary field of electrostatic polarisation, that is, for a field in which δD/δ_t_ = 0, there will be some change in the polarisation of the body due to its motion, given by _u__{_x_}(δD/δ_x_). Hence we must add this term to a purely temporal rate of change δD/δ_t_. Doing this we immediately arrive at equations (1·21) and (2·21) for the special case considered there. Thus the Hertz-Heaviside form of field equations gives _unity_ as the value for the Fresnelian convection co-efficient. It has been shown in the historical introduction how this is entirely at variance with the observed optical facts. As a matter of fact, Larmor has shown (Aether and Matter) that 1 - 1/μ² is not only sufficient but is also necessary, in order to explain experiments of the Arago prism type. A short summary of the electromagnetic experiments bearing on this question, has already been given in the introduction. According to Hertz and Heaviside the total polarisation is situated in the medium itself and is completely carried away by it. Thus the electromagnetic effect outside a moving medium should be proportional to K, the specific inductive capacity. _Rowland_ showed in 1876 that when a charged condenser is rapidly rotated (the dielectric remaining stationary), the magnetic effect outside is proportional to K, the Sp. Ind. Cap. _Röntgen_ (Annalen der Physik 1888, 1890) found that if the dielectric is rotated while the condenser remains stationary, the effect is proportional to K - 1. _Eichenwald_ (Annalen der Physik 1903, 1904) rotated together both condenser and dielectric and found that the magnetic effect was proportional to the potential difference and to the angular velocity, but was completely independent of K. This is of course quite consistent with Rowland and Röntgen. _Blondlot_ (Comptes Rendus, 1901) passed a current of air in a steady magnetic field H_{_y_}, (H = H_{_z_} = 0). If this current of air moves with velocity _u__{_x_} along the _x_-axis, an electromotive force would be set up along the _z_-axis, due to the relative motion of matter and magnetic tubes of induction. A pair of plates at _z_ = ±_a_, will be charged up with density ρ = D_{_z_} = KE = K. _u__{_s_} H_{_y_}/c. But Blondlot failed to detect any such effect. _H. A. Wilson_ (Phil. Trans. Royal Soc. 1904) repeated the experiment with a cylindrical condenser made of ebony, rotating in a magnetic field parallel to its own axis. He observed a change proportional to K — 1 and not to K.