(I″) div D = ρ (IV″) curl E = - _d_B/_dt_, Div B = 0 (V′) Then for moving non-magnetised bodies, Lorentz puts (page 223, 3) μ = 1, B = H, and in addition to that takes account of the occurrence of the di-electric constant ε, and conductivity σ according to equations (ε_q_XXXIV″, p. 327) D - E = (ε - 1) {E + [_u_B]} (ε_q_XXXIII′, p. 223) J = σ(E + [_u_B]) Lorentz’s E, D, H are here denoted by E, M, _e_, _m_ while J denotes the conduction current. The three last equations which have been just cited here coincide with eqn (II), (III), (IV), the first equation would be, if J is identified with C, = _u_ρ (the current being zero for σ = 0, (29) Curl [H - (_u_, E)] = C + _d_D/_dt_ - curl [_u_D], and thus comes out to be in a different form than (1) here. Therefore for magnetised bodies, Lorentz’s equations do not correspond to the Relativity Principle. On the other hand, the form corresponding to the relativity principle, for the condition of non-magnetisation is to be taken out of (D) in §8, with μ = 1, not as B = H, as Lorentz takes, but as (30) B - [_u_D] = H - [_u_D] (M - [_u_E] = _m_ - [_ue_]. Now by putting H = B, the differential equation (29) is transformed into the same form as eqn (1) here when _m_ - [_ue_] = M - [_u_E]. Therefore it so happens that by a compensation of two contradictions to the relativity principle, the differential equations of Lorentz for moving non-magnetised bodies at last agree with the relativity postulate. If we make use of (30) for non-magnetic bodies, and put accordingly H = B + [_u_, (D - E)], then in consequence of (C) in §8, (ε - 1) (E + [_u_, B]) = D - E + [_u_. [_u_, D - E]], _i.e._ for the direction of _u_, (ε - 1) (E + [_u_B])_{_u_} = (D - E)_{_u_} and for a perpendicular direction ū, (ε - 1) [E + (_u_B)]_{_u_} = (1 - _u²_) (D - E)_{_u_} _i.e._ it coincides with Lorentz’s assumption, if we neglect _u²_ in comparison to 1. Also to the same order of approximation, Lorentz’s form for J corresponds to the conditions imposed by the relativity principle [comp. (E) § 8]—that the components of J_{_u_}, J_{_ū_} are equal to the components of σ (E + [_u_ B]) multiplied by √(1 - _u²_) or 1 / √(1 - _u²_) respectively. §10. Fundamental Equations of E. Cohn. E. Cohn assumes the following fundamental equations. (31) Curl (M + [_u_ E]) = _d_E/_dt_ + u div. E + J - Curl [E - (_u_. M)] = _d_M/_dt_ + u div. M. (32) J = σ E, = ε E - [_u_ M], M = μ (_m_ + [_u_ E.]) where E M are the electric and magnetic field intensities (forces), E, M are the electric and magnetic polarisation (induction). The equations also permit the existence of true magnetism; if we do not take into account this consideration, div. M. is to be put = 0.