was very great; and this meant that the electric intensity
would have to vary with extreme rapidity. The simplest means of obtaining a result of this kind would be to produce an oscillating field of electric intensity in which the oscillations were extraordinarily rapid, several billions a second. But no mechanical contrivance could yield such results, and apart from mechanical contrivances no other methods suggested themselves. For these reasons a number of years passed before Maxwell’s views could be put to the test.
In 1885 Helmholtz directed the attention of his pupil, Hertz, to the problem. Hertz was one of the most remarkable experimenters of the nineteenth century; he succeeded at last in vanquishing the technical difficulties and in generating by purely electrical means an oscillating electric field of extremely high frequency. Electromagnetic waves of sufficient intensity were thus produced; and after having been sidetracked for a time by a secondary phenomenon whose nature was elucidated by Poincaré, Hertz verified the fact that the waves advanced with the speed of light and indeed possessed all the essential properties of light waves other than those of visibility to the human eye. Thus, as a result of Hertz’s experiments, the foundations were laid for the commercial use of wireless and radio; but, more important still, Maxwell’s electromagnetic theory of light establishing the intimate connection between electricity and optics had at last been vindicated.
Just as Hertz had proved that electromagnetic waves were of the same nature as light waves, so now, conversely, Lebedew succeeded in showing that light waves were of the same nature as electromagnetic ones. It is, indeed, a necessary consequence of Maxwell’s theory that electromagnetic waves should exert a definite pressure on bodies upon which they impinge, and this pressure was verified and measured by Lebedew and again by Nichols and Hull in the case of light waves falling on matter.[42]
Yet it must be realised that marvellous as were Maxwell’s equations, so far as the field phenomena in empty space were concerned, they led to conflict with experiment in the presence of matter. The conflict was notably apparent when the phenomenon of dispersion was considered; and by dispersion we mean the splitting up of white light by a prism into a rainbow of colors. Maxwell’s equations proved that on entering a transparent dielectric such as glass, the electromagnetic vibrations which constitute light would be slowed down; and this slowing down, as had long been known, would account for refraction. But dispersion demands that the angle of refraction, hence the slowing down, should depend on the frequency, hence on the colour of the light vibrations; and this is what Maxwell’s theory failed to account for. His equations showed that the slowing down would be the same for all frequencies. A similar difficulty was encountered when the phenomenon of the magnetic rotation of the plane of polarisation was considered. Faraday had discovered the existence of this phenomenon, and any theory of electromagnetics would have to account for it. Nevertheless, it will be seen that all these difficulties which beset Maxwell’s theory arose solely when experiments involving matter were taken into consideration. It appeared, therefore, that the fault resided not in the field equations themselves, but in the too crude assumptions that had been made with respect to the constitution of matter.
Furthermore, Maxwell had considered more especially the case of matter at rest in the ether, and it was necessary to investigate the general case of matter in motion. This extension of Maxwell’s theory, known as the electrodynamics of moving bodies, was attempted by Hertz, who assumed that matter in motion through the ether would drag the ether along with it in its interior—a hypothesis not so radical, but somewhat in line with that of Stokes. So long as conductors were contemplated, Hertz’s hypothesis seemed to be in accord with experiment; but when we came to consider dielectrics in motion, Hertz’s anticipations were contradicted. Fizeau’s experiment in particular, proving that the velocity of running water did not compound with the velocity of the light in stagnant water, was incompatible with Hertz’s hypothesis of a total drag. Also, in an experiment of a totally different nature, dealing with the motion of a dielectric through a magnetic field, Wilson proved the existence of the same partial drag, given once again by Fresnel’s convection coefficient. It appeared, therefore, that Fresnel’s convection coefficient was a fact which would have to be fitted into any theoretical scheme of the electrodynamics of moving dielectrics.
Now, Fresnel’s partial drag hypothesis, though seemingly confirmed by experiment, had yet to be accounted for theoretically, and this appeared impossible so long as we remained in such utter ignorance of the ultimate constitution of dielectrics and of matter in general. At this stage Lorentz started his theoretical investigations which were to lead him to a refinement of Maxwell’s equations, extending their application to cases where matter was present, either at rest or in motion through the ether. As we have mentioned, Maxwell’s equations had proved themselves incapable of accounting for dispersion. It appeared necessary to conceive of some structure for dielectrics which would act selectively, imposing different degrees of retardation on light waves of different frequencies. Lorentz achieved this result by assuming that electricity was atomic and that matter was constituted by more or less complicated groupings of these electric atoms or electrons.
In the case of dielectrics or non-conductors such as glass, the electrons were assumed to be tied down to their atoms by elastic forces. When disturbed they would vibrate with some characteristic frequency depending on the magnitude of the elastic forces which held them in place. If, then, a wave of light were transmitted through the transparent dielectric, the electrons would be set into forced vibration and the retarding effect these vibrations would impose on the incident light would depend on the relative frequencies of the incident light and of the natural frequencies of the electrons. In this way it was possible to obtain a mathematical interpretation of dispersion.