"Now, although the plans of the edifice of the electromagnetic theory of light were laid in 1880 by H. A. Lorentz, and even indicated much earlier by W. Weber, a full 10 years were required before the discoveries of Heinrich Hertz gave the impetus to collect the building stones and work them into shape. In the years 1890-93 a number of works appeared by F. Richarz, H. Ebert and G. Johnstone Stoney, mostly dealing with the mechanism of the emission of luminous vapours, and in which attempts are made, on the basis of the kinetic theory of gases, to determine the magnitude of the elementary electrical quantity, called by Stoney by the now universally accepted name of electron. The result of these calculations is important, as showing that it does not clash with other experimental data. Thus, H. Ebert proved that the amplitude of an electron in luminous sodium vapour need only be a small fraction of a molecular diameter in order to excite a radiation of the absolute intensity determined by E. Wiedemann. The way of determining the amount of electricity contained in the electron is very simple. The quantity of electricity required for the electrolytic evolution of 1 cubic cm. of any monatomic gas is divided by Loschmidt's number—i.e., the number of gas molecules contained in 1 cubic cm. Considering the uncertainty of this number, it can only be said that one electron contains about 10-10 electrostatic units. The value of this number would be very questionable but for the circumstance that a whole series of other methods... tend to very similar values. While it thus became clear that the supposition of vibrating ionic charges was compatible with observed phenomena as regards the order of magnitude, two works appeared... which completed the edifice of the electromagnetic theory of light. Of these works, that of Helmholtz only deals with the special question of the dispersion of light in absorbing media, while the other, due to H. A. Lorentz, goes much further. It shows how the assumption of vibrating charged particles in transparent bodies eliminates all the difficulties in the way of an adequate explanation of the propagation of light in moving bodies, such as the aberration of stellar light. Lorentz's theory leaves Maxwell's theory untouched as regards the free ether. A material body influences the optical and electrical processes only by virtue of the electric charges contained in it, while in the interspaces filled with ether everything remains unhanged. Maxwell's "dielectric constant" therefore disappears as a fundamental conception in Lorentz's theory. It becomes a derived conception, and it is immediately seen that for rapid electric oscillations, in which the inertia of the vibrating charges enters into consideration, it has no significance. The same applies mutatis mutandis, to the magnetic permeability."