In contradistinction to Hertz, Lorentz assumed that the ether was never disturbed by the motion of matter; and just as in Maxwell’s theory, a luminous source in motion through the ether would never communicate its velocity to the light waves it emitted. With the constitution of matter postulated in this way, Lorentz set out to extend Maxwell’s theory of electromagnetics in the free ether to cases where matter was present, either at rest or in motion. As already mentioned, he succeeded in giving a theory of dispersion, one of the phenomena which Maxwell had been unable to explain. Furthermore, such highly complicated phenomena as abnormal dispersion, absorption, metallic reflection, selective reflection, body colour and many others were accounted for by the theory with great precision. The apparent partial drag of the ether verified in Fizeau’s experiment and also in Wilson’s (Fresnel’s convection coefficient) was proved to be a necessary consequence of the theory. It had nothing to do with a partial dragging of the ether, since the ether was in no wise disturbed by the motion of the matter; the apparent drag was accounted for solely in terms of the electronic constitution of the moving matter. In this way Fresnel’s convection coefficient received the rational explanation which it had thus far lacked. The most refined experiments confirmed Lorentz’s theory in a number of minute details; and when the electron was finally isolated and studied, all doubts appeared to be removed, at least as to the general correctness of the doctrine. Theories, however, are especially convincing, not so much when they account for what has been observed but has not been explained, as when they allow us to foretell phenomena which no one has anticipated. Triumphs of this sort were soon forthcoming in the case of Lorentz’s theory. For instance, the theory required that light waves be produced by electrons vibrating in the interior of the atom. Inasmuch as the presence of a magnetic or an electric field affects the motion of electrified bodies, the vibrations of the electrons in the luminous atom should be modified by the presence of a strong magnetic field. Calculation then showed Lorentz that a magnetic field would cause an atom normally emitting a monochromatic light to emit two or three separate lights, according to the relative inclination of the magnetic field to the line of sight of the observer. This totally unexpected effect was soon verified by Zeeman, one of Lorentz’s colleagues, and is now known as the Zeeman effect. It has since been observed in the light emitted by sunspots, proving that very intense magnetic fields must there be present.